# [RFC] [Discussion] Serializable closures from constant expressions

- Thread: https://externals.io/message/131198
- Original archive: https://news-web.php.net/php.internals/131198
- Started by Nicolas Grekas on 2026-06-10T17:02:06+00:00
- Messages: 15
- RFC: Serializable closures, by reference to the code that declares them (Under discussion) — https://externals.io/rfc/serializable_closures.md
- Reply by email to internals@lists.php.net
- More for agents: search with https://externals.io/search.md?q={query} · RFCs at https://externals.io/rfc.md · JSON API at https://externals.io/api · documentation at https://externals.io/llms.txt

---

## #131198 — Nicolas Grekas — 2026-06-10T17:02:06+00:00

URL: https://externals.io/message/131198#131198

Hi,

I'd like to open the discussion on a new RFC:
https://wiki.php.net/rfc/serializable_closures

PHP 8.5 allowed closures in attribute arguments and parameter defaults.
These closures are static and capture nothing, yet they cannot be
serialized, which silently breaks every `serialize()`-based metadata cache
that meets them.

The RFC proposes to make them serializable as references to their
declaration site.

Feedback welcome.

Cheers,
Nicolas

---

## #131296 — tim@bastelstu.be — 2026-06-15T19:30:18+00:00

URL: https://externals.io/message/131198#131296 · In reply to: #131198

Hi


> I'd like to open the discussion on a new RFC:
> https://wiki.php.net/rfc/serializable_closures

Don't forget to link the discussion thread in the RFC. For your 
convenience: https://news-web.php.net/php.internals/131198

As to the RFC itself: I think it might be useful to split this into two 
RFCs, similarly to how Volker and I split the initial support for 
Closures in const-expr into support for Closures and support for first 
class callables.

Specifically I believe that the proposed serialization format is fragile 
and the stated security model, which I suspect heavily influenced the 
design of the serialization format, is probably overly cautious.

I don't think it is a problem to make `unserialize()` a Closure factory, 
because the created Closure is “inert”. Contrary to arbitrary object 
unserialization (which will immediately call the deserialization hooks 
and then later __destruct()), the Closure will not actually do anything 
unless it is called.

If folks are able to unserialize arbitrary payloads - which is 
documented to be unsafe - they already have capabilities that are much 
more powerful than “creating Closures”.

For first class callables specifically, there is a very obvious and 
stable identifier to use: The underlying function name.

For regular Closures, I don't like how changing something unrelated 
(namely adding new Closures to a class) can change the meaning of the 
serialized data. This is not how serialization works right now: The 
value in question is in full control of its serialization format and is 
able to “gracefully” support existing serialized payloads using an old 
format within an unserialization hook (e.g. by including a version 
number field). This allows to e.g. keep serialized queue jobs working 
across deploys.

As for the `var_export()` in the future scope: I think adding support for 
first class callables to `var_export()` would be a change that can just 
be done without an RFC. It might be a good first step that might already 
be helpful to your use case?

Best regards
Tim Düsterhus

---

## #131753 — Nicolas Grekas — 2026-07-04T08:47:04+00:00

URL: https://externals.io/message/131198#131753 · In reply to: #131296

Hi,

thanks for having a look

Le lun. 15 juin 2026 à 21:30, Tim Düsterhus <tim@bastelstu.be> a écrit :

> Hi
>
> 
> > […]
>
> Don't forget to link the discussion thread in the RFC. For your
> convenience: https://news-web.php.net/php.internals/131198
>
> As to the RFC itself: I think it might be useful to split this into two
> RFCs, similarly to how Volker and I split the initial support for
> Closures in const-expr into support for Closures and support for first
> class callables.
>

The 8.5 split worked because closures and FCCs were separable features.
Here it's one mechanism, the FCC half alone or the anonymous-closure one
alone would be just missing its other half.
I'd keep it as one RFC.



> Specifically I believe that the proposed serialization format is fragile
> and the stated security model, which I suspect heavily influenced the
> design of the serialization format, is probably overly cautious.
>
> I don't think it is a problem to make `unserialize()` a Closure factory,
> because the created Closure is “inert”. Contrary to arbitrary object
> unserialization (which will immediately call the deserialization hooks
> and then later __destruct()), the Closure will not actually do anything
> unless it is called.
>
> If folks are able to unserialize arbitrary payloads - which is
> documented to be unsafe - they already have capabilities that are much
> more powerful than “creating Closures”.
>

Creation is inert, but the point of these payloads is to be called. Once
it's called, the only thing that matters is which callables a payload can
name.
Declared-set resolution: an attacker can swap one closure the class already
declares for another.
Unrestricted name-based: {function: "system"} with attacker-supplied args
is a call gadget shipped in the engine, against any app that unserializes
untrusted input.
"allowed_classes" exists and 8.6 tightened session defaults precisely
because we bound damage despite unserialize being documented unsafe.
So I'd keep the declared-set boundary as required defense-in-depth /
hardening.



> For first class callables specifically, there is a very obvious and
> stable identifier to use: The underlying function name.
>
> For regular Closures, I don't like how changing something unrelated
> (namely adding new Closures to a class) can change the meaning of the
> serialized data. This is not how serialization works right now: The
> value in question is in full control of its serialization format and is
> able to “gracefully” support existing serialized payloads using an old
> format within an unserialization hook (e.g. by including a version
> number field). This allows to e.g. keep serialized queue jobs working
> across deploys.
>

Agreed: first-class callables now serialize with the function name as
identifier, no ordinal involved. The id is `member@callable`, e.g.
`$billingAddress@Order::isStrict` for `#[When(self::isStrict(...))]` on
that property, `$p@strlen` for a plain function. The member prefix keeps
resolution local to one reflection element instead of scanning the whole
class on every cache read (fat classes would pay otherwise), and it gives
both closure forms the same staleness rule: a reference is valid while its
member and its declaration survive. Adding, removing or reordering anything
else in the class changes nothing; renaming the target method fails.

One catch: the idiomatic form references a private helper of the same
class, #[When(self::isStrict(...))], and Closure::fromCallable('C::priv')
from global scope throws "cannot access private method". So resolution
doesn't resolve the name directly: it checks if the named member declares
that exact reference, then evaluates the declaration in class scope, name
as address, declaration as guard.
Private keeps working, {..., "$p@system"} stays rejected because no class
declares it.



> As for the `var_export()` in the future scope: I think adding support for
> first class callables to `var_export()` would be a change that can just
> be done without an RFC. It might be a good first step that might already
> be helpful to your use case?
>

Not really helpful for the main use cases I gathered, which require full
compat with serialize semantics, but yeah, I agree this can be dealt with
on its own.

RFC updated!
I tried to reflect all your points in the update.

Cheers,
Nicolas

---

## #131754 — Nicolas Grekas — 2026-07-04T09:02:38+00:00

URL: https://externals.io/message/131198#131754 · In reply to: #131753

Le sam. 4 juil. 2026 à 10:47, Nicolas Grekas <nicolas.grekas+php@gmail.com>
a écrit :

> Hi,
>
> thanks for having a look
>
> Le lun. 15 juin 2026 à 21:30, Tim Düsterhus <tim@bastelstu.be> a écrit :
>
> > […]
>
> The 8.5 split worked because closures and FCCs were separable features.
> Here it's one mechanism, the FCC half alone or the anonymous-closure one
> alone would be just missing its other half.
> I'd keep it as one RFC.
>
>
>
> > […]
>
> Creation is inert, but the point of these payloads is to be called. Once
> it's called, the only thing that matters is which callables a payload can
> name.
> Declared-set resolution: an attacker can swap one closure the class
> already declares for another.
> Unrestricted name-based: {function: "system"} with attacker-supplied args
> is a call gadget shipped in the engine, against any app that unserializes
> untrusted input.
> "allowed_classes" exists and 8.6 tightened session defaults precisely
> because we bound damage despite unserialize being documented unsafe.
> So I'd keep the declared-set boundary as required defense-in-depth /
> hardening.
>
>
>
> > […]
>
> Agreed: first-class callables now serialize with the function name as
> identifier, no ordinal involved. The id is `member@callable`, e.g.
> `$billingAddress@Order::isStrict` for `#[When(self::isStrict(...))]` on
> that property, `$p@strlen` for a plain function. The member prefix keeps
> resolution local to one reflection element instead of scanning the whole
> class on every cache read (fat classes would pay otherwise), and it gives
> both closure forms the same staleness rule: a reference is valid while its
> member and its declaration survive. Adding, removing or reordering anything
> else in the class changes nothing; renaming the target method fails.
>
> One catch: the idiomatic form references a private helper of the same
> class, #[When(self::isStrict(...))], and Closure::fromCallable('C::priv')
> from global scope throws "cannot access private method". So resolution
> doesn't resolve the name directly: it checks if the named member declares
> that exact reference, then evaluates the declaration in class scope, name
> as address, declaration as guard.
> Private keeps working, {..., "$p@system"} stays rejected because no class
> declares it.
>
>
>
> > […]
>
> Not really helpful for the main use cases I gathered, which require full
> compat with serialize semantics, but yeah, I agree this can be dealt with
> on its own.
>
> RFC updated!
> I tried to reflect all your points in the update.
>


Side-note to all:

I'd also be fine with a limited version of this RFC that'd remove the
serialize-related part and that'd keep only the proposed Reflection-based
API. This is the very core where engine support is needed. The serialize
part would make attributes work seamlessly with backends that use
 `serialize()`, but my use cases build on the deepclone/VarExporter
extension/components, and those need only reflection.

In case that can help bring a broader consensus.

Nicolas

---

## #131758 — Pierre — 2026-07-04T09:33:19+00:00

URL: https://externals.io/message/131198#131758 · In reply to: #131198

Le 10/06/2026 à 19:02, Nicolas Grekas a écrit :
> Hi,
>
> I'd like to open the discussion on a new RFC:
> https://wiki.php.net/rfc/serializable_closures
>
> PHP 8.5 allowed closures in attribute arguments and parameter 
> defaults. These closures are static and capture nothing, yet they 
> cannot be serialized, which silently breaks every 
> `serialize()`-based metadata cache that meets them.

Would this mean that if you serialize such object (in some cache for 
example) then change the object default value in code in between, deploy 
your new code version then load the outdated cache, your object would 
behave inconsistently, because the default value has been changed, but 
the loaded object would not honor the new default behavior ?

For caches it's not that much a problem because you mostly clear caches 
when you deploy, but considering, let's say for example, Symfony 
messenger messages, which are basically `serialize()`'d per default, it 
becomes a major problem.

--

Pierre

---

## #131759 — Pierre — 2026-07-04T09:37:36+00:00

URL: https://externals.io/message/131198#131759 · In reply to: #131198

Le 10/06/2026 à 19:02, Nicolas Grekas a écrit :
> Hi,
>
> I'd like to open the discussion on a new RFC:
> https://wiki.php.net/rfc/serializable_closures
>
> PHP 8.5 allowed closures in attribute arguments and parameter 
> defaults. These closures are static and capture nothing, yet they 
> cannot be serialized, which silently breaks every 
> `serialize()`-based metadata cache that meets them.
>
> The RFC proposes to make them serializable as references to their 
> declaration site.
>
> Feedback welcome.
>
> Cheers,
> Nicolas

Please forget my previous email, this was answered in the RFC:

 > Unserializing autoloads the class if needed and recreates the closure 
/as if its constant expression had just been evaluated/: same code, 
statically scoped to its declaring class (|self::|, private member 
access and |static::| behave identically),

Sorry for the noise!

Regards,

--

Pierre

---

## #131789 — Nicolas Grekas — 2026-07-07T11:52:47+00:00

URL: https://externals.io/message/131198#131789 · In reply to: #131754

Hi there,

Le sam. 4 juil. 2026 à 11:02, Nicolas Grekas <nicolas.grekas+php@gmail.com>
a écrit :

>
>
> Le sam. 4 juil. 2026 à 10:47, Nicolas Grekas <nicolas.grekas+php@gmail.com>
> a écrit :
>
> > […]
>
>
> Side-note to all:
>
> I'd also be fine with a limited version of this RFC that'd remove the
> serialize-related part and that'd keep only the proposed Reflection-based
> API. This is the very core where engine support is needed. The serialize
> part would make attributes work seamlessly with backends that use
> `serialize()`, but my use cases build on the deepclone/VarExporter
> extension/components, and those need only reflection.
>
> In case that can help bring a broader consensus.
>


I'd be happy to gather more feedback on this proposal. The PHP 8.6 deadline
is approaching fast and while this can wait, it's not substantial enough to
wait one more year IMHO.
Opening a vote without hearing that some consensus exists before is a
recipe for failure, so I'd rather send these reminders before opening one.
I get the topic is not as hype as generics or other syntax improvements,
but incremental improvements driven by userland needs is still how we built
a relevant engine ;)

Thanks!
Nicolas

---

## #131820 — tim@bastelstu.be — 2026-07-08T19:28:03+00:00

URL: https://externals.io/message/131198#131820 · In reply to: #131753

Hi

I have given the RFC another read before reading your email and the 
replies I'm giving below might possibly reflect that.


> > […]
> 
> The 8.5 split worked because closures and FCCs were separable features.
> Here it's one mechanism, the FCC half alone or the anonymous-closure 
> one
> alone would be just missing its other half.
> I'd keep it as one RFC.

I made that suggestion, because I believe that building a robust 
solution for first class callables is much easier than for “full 
Closures”.

While reading the RFC initially and now the updated version, I got the 
feeling that it was “overfitted” to solve the specific use case and 
deployment scenario that you consider a “best practice”, which I feel 
results in “weird” behavior when one leaves that happy path. The updated 
version is already better, particularly around first class callables, 
but I can't say that I'm *happy* with it.

> > […]
> 
> Creation is inert, but the point of these payloads is to be called. 
> Once
> it's called, the only thing that matters is which callables a payload 
> can
> name.

Yes, but this requires application code to already be set up to try to 
perform a function call on unserialized data, which is not a typical 
situation. The typical “unserialization to RCE” vector looks something 
like this:

     $sessionData = unserialize($_COOKIE['session']);

where the cookie contains a serialized payload. This looks totally 
reasonable unless you know that `unserialize()` will already execute 
arbitrary code by itself. For Closures in the payload to be exploitable 
you need extra code that is much less likely to be written “by accident” 
in situations where untrusted data ends up in unserialize.

And then the attacker also needs to control the inputs to the Closure in 
question for this to actually be exploitable. Even if a Closure created 
from unserialized data is called, the code performing the call expects a 
function matching a specific signature, that is unlikely to be matched 
by “exploitable” functionality. Calling `system()` without any arguments 
will just fail. And the same is true if `system()` ends up in a place 
where the caller passes an object as the first parameter.

> So I'd keep the declared-set boundary as required defense-in-depth /
> hardening.

That said, I think I'd be okay with keeping this defense-in-depth for 
now, but I think I would then want a serialization format that allows 
for a clean “forward compatibility” in case we want to relax this later. 
Specifically I think we should be careful with “reserving” this many 
top-level keys with specific names in the serialization payload. As an 
example, the `class` key makes adding support for the future-scope 
“free-function attributes” unnecessarily complicated, which is part of 
what I meant by “overfitted to the use-case” above.

An idea that I have not given too much thought would be making the 
serialization payload a “tagged union” with something like 
`[get_mangled_object_vars($this), ["const-expr", ['Order', 
'billingAddress@0']]]`. This could then be extended to a 
`[get_mangled_object_vars($this), ["fcc", ["strrev"]]]` or similar. 
Perhaps seek inspiration from the serialization format for ext/uri or 
ext/random, which have explicitly been designed to be able to be 
extended in a backwards compatible fashion and to avoid naming clashes 
between actual userland properties and internal state.

> Agreed: first-class callables now serialize with the function name as
> identifier, no ordinal involved. The id is `member@callable`, e.g.
> `$billingAddress@Order::isStrict` for `#[When(self::isStrict(...))]` on
> that property, `$p@strlen` for a plain function. The member prefix 
> keeps
> resolution local to one reflection element instead of scanning the 
> whole
> class on every cache read (fat classes would pay otherwise), and it 
> gives
> both closure forms the same staleness rule: a reference is valid while 
> its
> member and its declaration survive. Adding, removing or reordering 
> anything
> else in the class changes nothing; renaming the target method fails.

I wonder if using the function name would also work for regular 
Closures. I've updated the names for PHP 8.4 to include the name of the 
declaring scope and line number, which is exactly the information the 
RFC is already using as a guard as of now.

That said: I understand that accurately identifying “full Closures” is 
complicated, but I also feel that the line number guard for proper 
Closures is making serialization payloads fragile across deployments. 
Adding a single “use” import at the top of the file would break all 
unserialization - which again feels like the overfitting. I don't have a 
good suggestion here, which is why I suggested to solve the first class 
callable and “full Closure” cases separately.

> One catch: the idiomatic form references a private helper of the same
> class, #[When(self::isStrict(...))], and 
> Closure::fromCallable('C::priv')
> from global scope throws "cannot access private method". So resolution
> doesn't resolve the name directly: it checks if the named member 
> declares
> that exact reference, then evaluates the declaration in class scope, 
> name
> as address, declaration as guard.

Yes, unserialization ignores or “trusts” visibility. That's just how it 
works.

> > […]
> 
> Not really helpful for the main use cases I gathered, which require 
> full
> compat with serialize semantics, but yeah, I agree this can be dealt 
> with
> on its own.

Specifically with regard to `var_export()`, I'm also concerned about the 
ReflectionFunction::getConstExprId(), 
ReflectionFunction::getConstExprClass(), and Closure::fromConstExpr() 
additions, which very much feel like “exposing an implementation 
detail”. These functions will be part of the public API and 
documentation, which means that users will come across them. I don't 
currently see how we can meaningfully document them, since they serve 
such a narrow use case based on very specific assumptions.

Best regards
Tim Düsterhus

---

## #131821 — tim@bastelstu.be — 2026-07-08T19:39:56+00:00

URL: https://externals.io/message/131198#131821 · In reply to: #131754

Hi


> I'd also be fine with a limited version of this RFC that'd remove the
> serialize-related part and that'd keep only the proposed 
> Reflection-based
> API. This is the very core where engine support is needed. The 
> serialize
> part would make attributes work seamlessly with backends that use
> `serialize()`, but my use cases build on the deepclone/VarExporter
> extension/components, and those need only reflection.
> 
> In case that can help bring a broader consensus.

As indicated in my previous email, I'm also concerned about the 
Reflection-based API and don't consider it good language design. Given 
that the Reflection-based API needs Reflection and the specific 
constraint of “the serialized payload is only valid until the next 
deployment”, I also don't see how it would enable anything that you 
can't already do.

- For public first class callables, just generate a first class 
callable.
- For private and protected ones generate `(new ReflectionMethod($class, 
$method))->getClosure()`.
- For full Closures generate the the appropriate Reflection chain 
accessing the right Closure, e.g. `(new ReflectionProperty($class, 
$method))->getAttributes()[$attrNo]->getArguments()[$argumentNo]`.

The proposed new API would effectively do the same, and then add the 
extra safety checks that you, from what I understand, wouldn't need for 
this use case.

Best regards
Tim Düsterhus

---

## #131822 — tim@bastelstu.be — 2026-07-08T19:53:27+00:00

URL: https://externals.io/message/131198#131822 · In reply to: #131789

Hi


> Opening a vote without hearing that some consensus exists before is a
> recipe for failure, so I'd rather send these reminders before opening 
> one.
> I get the topic is not as hype as generics or other syntax 
> improvements,
> but incremental improvements driven by userland needs is still how we 
> built
> a relevant engine ;)

Despite my concerns regarding the proposal as-written, I can only agree 
with this specific message. I find this kind of boring building block 
functionality that you wouldn’t notice because it “just works” to be 
much more impactful than some of the fancy headline features that make 
rounds on social media. But the “just works” is a constraint, as with 
any other RFC we need to get it *right* due to backwards compatibility 
considerations. And of course it shouldn't just work for one situation, 
but be reasonably generically applicable. This is really not something 
that should be discussed between just Nicolas and me :-)

Best regards
Tim Düsterhus

---

## #131874 — Nicolas Grekas — 2026-07-11T22:13:51+00:00

URL: https://externals.io/message/131198#131874 · In reply to: #131821

Thanks Tim,

replying to both your messages in one go:


Le mer. 8 juil. 2026 à 21:28, Tim Düsterhus <tim@bastelstu.be> a écrit :

> Hi
>
> I have given the RFC another read before reading your email and the
> replies I'm giving below might possibly reflect that.
>
> 
> > […]
>
> I made that suggestion, because I believe that building a robust
> solution for first class callables is much easier than for “full
> Closures”.
>
> While reading the RFC initially and now the updated version, I got the
> feeling that it was “overfitted” to solve the specific use case and
> deployment scenario that you consider a “best practice”, which I feel
> results in “weird” behavior when one leaves that happy path. The updated
> version is already better, particularly around first class callables,
> but I can't say that I'm *happy* with it.
>
> > […]
>
> Yes, but this requires application code to already be set up to try to
> perform a function call on unserialized data, which is not a typical
> situation. The typical “unserialization to RCE” vector looks something
> like this:
>
>      $sessionData = unserialize($_COOKIE['session']);
>
> where the cookie contains a serialized payload. This looks totally
> reasonable unless you know that `unserialize()` will already execute
> arbitrary code by itself. For Closures in the payload to be exploitable
> you need extra code that is much less likely to be written “by accident”
> in situations where untrusted data ends up in unserialize.
>
> And then the attacker also needs to control the inputs to the Closure in
> question for this to actually be exploitable. Even if a Closure created
> from unserialized data is called, the code performing the call expects a
> function matching a specific signature, that is unlikely to be matched
> by “exploitable” functionality. Calling `system()` without any arguments
> will just fail. And the same is true if `system()` ends up in a place
> where the caller passes an object as the first parameter.
>
> > […]
>
> That said, I think I'd be okay with keeping this defense-in-depth for
> now, but I think I would then want a serialization format that allows
> for a clean “forward compatibility” in case we want to relax this later.
> Specifically I think we should be careful with “reserving” this many
> top-level keys with specific names in the serialization payload. As an
> example, the `class` key makes adding support for the future-scope
> “free-function attributes” unnecessarily complicated, which is part of
> what I meant by “overfitted to the use-case” above.
>

Happy we converged on this defense-in-depth aspect, I wouldn't be
comfortable with an unbounded allowance for closures.

An idea that I have not given too much thought would be making the
> serialization payload a “tagged union” with something like
> `[get_mangled_object_vars($this), ["const-expr", ['Order',
> 'billingAddress@0']]]`. This could then be extended to a
> `[get_mangled_object_vars($this), ["fcc", ["strrev"]]]` or similar.
> Perhaps seek inspiration from the serialization format for ext/uri or
> ext/random, which have explicitly been designed to be able to be
> extended in a backwards compatible fashion and to avoid naming clashes
> between actual userland properties and internal state.
>

Thanks for the idea, I made it to the RFC + implementation.


> > […]
>
> I wonder if using the function name would also work for regular
> Closures. I've updated the names for PHP 8.4 to include the name of the
> declaring scope and line number, which is exactly the information the
> RFC is already using as a guard as of now.
>
> That said: I understand that accurately identifying “full Closures” is
> complicated, but I also feel that the line number guard for proper
> Closures is making serialization payloads fragile across deployments.
> Adding a single “use” import at the top of the file would break all
> unserialization - which again feels like the overfitting. I don't have a
> good suggestion here, which is why I suggested to solve the first class
> callable and “full Closure” cases separately.
>

I tweaked the line number to make it relative to the class, so that adding
a `use` doesn't invalidate payloads.
What we're building here is an addressing scheme and it's part of the
consensus we need to have in this RFC.
The one proposed here is quite efficient to implement and effective to
achieve the goal.

Also, no need for something as robust as a named identifier, it's really
fine that these payloads stay valid only for the code revision they were
generated with.



> > […]
>
> Yes, unserialization ignores or “trusts” visibility. That's just how it
> works.
>
> > […]
>
> Specifically with regard to `var_export()`, I'm also concerned about the
> ReflectionFunction::getConstExprId(),
> ReflectionFunction::getConstExprClass(), and Closure::fromConstExpr()
> additions, which very much feel like “exposing an implementation
> detail”. These functions will be part of the public API and
> documentation, which means that users will come across them. I don't
> currently see how we can meaningfully document them, since they serve
> such a narrow use case based on very specific assumptions.


see below

Le mer. 8 juil. 2026 à 21:39, Tim Düsterhus <tim@bastelstu.be> a écrit :

> 
> > […]
>
> As indicated in my previous email, I'm also concerned about the
> Reflection-based API and don't consider it good language design. Given
> that the Reflection-based API needs Reflection and the specific
> constraint of “the serialized payload is only valid until the next
> deployment”, I also don't see how it would enable anything that you
> can't already do.
>
> - For public first class callables, just generate a first class
> callable.
> - For private and protected ones generate `(new ReflectionMethod($class,
> $method))->getClosure()`.
> - For full Closures generate the the appropriate Reflection chain
> accessing the right Closure, e.g. `(new ReflectionProperty($class,
> $method))->getAttributes()[$attrNo]->getArguments()[$argumentNo]`.
>

What's missing from your analysis is the "provenance" metadata that
reflection currently gives no access to. I even had to add a flag in the C
struct to persist this. Nothing today distinguishes a closure that a class
declared in a constant expression from one built at runtime, yet that is
precisely the question a cache layer must answer before it may reference a
closure at all.

The userland deepclone extension and its pure-PHP polyfill are the
existence proof: both need this and I had to instrument
''ReflectionAttribute'' by overloading the class in the ext, and the
polyfill needs complex and fragile heuristic machinery. The RFC would make
these go away.

The proposed new API would effectively do the same, and then add the
> extra safety checks that you, from what I understand, wouldn't need for
> this use case
>


RFC updated.

Cheers,
Nicolas

---

## #131914 — Nicolas Grekas — 2026-07-14T10:16:38+00:00

URL: https://externals.io/message/131198#131914 · In reply to: #131874

Le dim. 12 juil. 2026 à 00:13, Nicolas Grekas <nicolas.grekas+php@gmail.com>
a écrit :

> Thanks Tim,
>
> replying to both your messages in one go:
>
>
> Le mer. 8 juil. 2026 à 21:28, Tim Düsterhus <tim@bastelstu.be> a écrit :
>
> > […]
>
> Happy we converged on this defense-in-depth aspect, I wouldn't be
> comfortable with an unbounded allowance for closures.
>
> An idea that I have not given too much thought would be making the
> > […]
>
> Thanks for the idea, I made it to the RFC + implementation.
>
>
> > […]
>
> I tweaked the line number to make it relative to the class, so that adding
> a `use` doesn't invalidate payloads.
> What we're building here is an addressing scheme and it's part of the
> consensus we need to have in this RFC.
> The one proposed here is quite efficient to implement and effective to
> achieve the goal.
>
> Also, no need for something as robust as a named identifier, it's really
> fine that these payloads stay valid only for the code revision they were
> generated with.
>

Following an off-list review + suggestion by Arnaud, I added a hash of
closures's body to the identifier.
This hash serves as a staleness check and replaces the line number.
It's stored at compile-time in the AST, and compared when unserializing.
The site@rank next to the hash remains so that polyfill/ext can be written
to help transitioning.




>
>
>
> > […]
>
>
> see below
>
> Le mer. 8 juil. 2026 à 21:39, Tim Düsterhus <tim@bastelstu.be> a écrit :
>
> > […]
>
> What's missing from your analysis is the "provenance" metadata that
> reflection currently gives no access to. I even had to add a flag in the C
> struct to persist this. Nothing today distinguishes a closure that a
> class declared in a constant expression from one built at runtime, yet
> that is precisely the question a cache layer must answer before it may
> reference a closure at all.
>
> The userland deepclone extension and its pure-PHP polyfill are the
> existence proof: both need this and I had to instrument
> ''ReflectionAttribute'' by overloading the class in the ext, and the
> polyfill needs complex and fragile heuristic machinery. The RFC would make
> these go away.
>
> The proposed new API would effectively do the same, and then add the
> > […]
>
>
> RFC updated.
>
> Cheers,
> Nicolas

---

## #131924 — Ilija Tovilo — 2026-07-14T22:18:30+00:00

URL: https://externals.io/message/131198#131924 · In reply to: #131198

Hi Nicolas


> I'd like to open the discussion on a new RFC:
> https://wiki.php.net/rfc/serializable_closures

Thank you for your RFC. Sorry for being late to the party.

I'm trying to understand the premise of the RFC. Why does Symfony need 
to cache attributes at all? Attribute construction is cheap, at least 
when compared to restoring them using `unserialize()` or even Symfony's 
VarExporter. Is this an artifact from the doc block attributes era when 
parsing them was expensive, or is there a practical reason today?

I found the RFC quite hard to read. It uses a lot of ambiguous 
terminology, sentences are long and rarely explained with an example. 
It's also very technical, though maybe necessarily so.

 > When the class's source changes, a stored reference either stops 
resolving or is rejected by the hash check; both throw an Exception on 
 `unserialize()`, which cache layers already treat as a miss.

I suppose Symfony handles `unserialize()` errors gracefully, but an 
 `unserialize()` call today will not fail unless an obvious BC break was 
made, e.g. by removing a serialized class. This is much more predictable 
than shifting some elements around or tweaking a closure implementation. 
I'm not convinced all caching layers today handle errors gracefully.

 > Security model

IMO the security argument for `unserialize()` is redundant. `unserialize()` 
is documented to be unsafe when called with unsanitized inputs. For 
php-src, we close all security reports related to `unserialize()` as invalid.

https://www.php.net/manual/en/function.unserialize.php

 > Do not pass untrusted user input to `unserialize()` regardless of the 
options value of allowed_classes. Unserialization can result in code 
being loaded and executed due to object instantiation and autoloading, 
and a malicious user may be able to exploit this.

Hence, I think security is a good reason for making the serialized 
format more complex than it needs to be.

The format is very complex and deals with a ton of edge cases, e.g. 
handling so many attribute target types. It's mostly hidden from users, 
but not completely (referring to the reflection API).

The implementation is also very large and complex.

Overall, I'm sadly not in favor of this RFC.

Ilija

---

## #131939 — Nicolas Grekas — 2026-07-15T15:52:51+00:00

URL: https://externals.io/message/131198#131939 · In reply to: #131924

Thanks for having a look

Le mer. 15 juil. 2026 à 00:20, Ilija Tovilo <tovilo.ilija@gmail.com> a
écrit :

> Hi Nicolas
>
> 
> > […]
>
> Thank you for your RFC. Sorry for being late to the party.
>
> I'm trying to understand the premise of the RFC. Why does Symfony need
> to cache attributes at all? Attribute construction is cheap, at least
> when compared to restoring them using `unserialize()` or even Symfony's
> VarExporter. Is this an artifact from the doc block attributes era when
> parsing them was expensive, or is there a practical reason today?
>

Attributes are one metadata source among several, in systems designed
around loaders: Doctrine entity metadata, Symfony Serializer mappings,
Validator constraints, to name a few widely used ones. Loaders read XML,
YAML, phpdoc in its day, attributes now, and produce a common
representation, merged and indexed along the way. That derived
representation is what goes into cache layers or gets dumped as PHP (a
cache layer too).

So the comparison is not newInstance() vs `unserialize()` for one attribute:
it is re-running the whole load+merge+index pipeline on every request vs
one cache fetch. https://github.com/symfony/symfony/issues/63228 describes
how closures in attributes disrupt this: the engine gives no way to put
them in a cache (via `serialize()` or exported PHP), so affected classes fall
back to runtime extraction. What you suggest is what is implemented today,
and it is the measured slow path. This RFC closes the gap by letting the
already-optimized representation be cached.



> I found the RFC quite hard to read. It uses a lot of ambiguous
> terminology, sentences are long and rarely explained with an example.
> It's also very technical, though maybe necessarily so.
>

Concrete pointers to unclear spots would help, I'll happily rework them.
The short description on the PR may be a better entry point:
https://github.com/php/php-src/pull/22716. The RFC is long because it maps
the problem space and the alternatives I explored; that context felt
necessary to judge the choices.



> > […]
> resolving or is rejected by the hash check; both throw an Exception on
> `unserialize()`, which cache layers already treat as a miss.
>
> I suppose Symfony handles `unserialize()` errors gracefully, but an
> `unserialize()` call today will not fail unless an obvious BC break was
> made, e.g. by removing a serialized class. This is much more predictable
> than shifting some elements around or tweaking a closure implementation.
> I'm not convinced all caching layers today handle errors gracefully.
>

The failure set is narrower than that. References are scoped to their
declaring element and carry a hash of the closure's code: shifting elements
around, adding methods, reordering, moving the class in the file, none of
that invalidates anything. A reference fails when its own closure's code
changed, when closures were added/removed in its own element, or when the
element or class was renamed. And when the closure's code changed, failing
is the correct outcome: the cached metadata was derived from the old code,
so the entry must be rebuilt, not partially reused.

Also, today's behavior on the comparable event (a renamed class) is
arguably worse than an exception: __PHP_Incomplete_Class propagates
silently until first use. Caches cannot return data they fail to restore
per PSR-6/16; treating a failed unserialize as a miss is how conforming
implementations behave.



> > […]
>
> IMO the security argument for `unserialize()` is redundant. `unserialize()`
> is documented to be unsafe when called with unsanitized inputs. For
> php-src, we close all security reports related to `unserialize()` as invalid.
>
> https://www.php.net/manual/en/function.unserialize.php



 > Do not pass untrusted user input to `unserialize()` regardless of the
> options value of allowed_classes. Unserialization can result in code
> being loaded and executed due to object instantiation and autoloading,
> and a malicious user may be able to exploit this.
>
> Hence, I think security is a good reason for making the serialized
> format more complex than it needs to be.


We did add allowed_classes, and we do defer __wakeup()/__unserialize()
calls until the whole payload is reconstructed, both because some gadget
surfaces were too risky despite that doc note. Security is not black and
white; hardening is about bounding consequences when users err. In Symfony
we also reject gadget chains as non-CVE, and we still fix them as
hardening. See https://github.com/ambionics/phpggc/tree/master/gadgetchains
for how effective chains are in practice.

The magnitude here is specific. Today a chain needs app-specific gadget
classes; that is why phpggc is a per-framework catalog. Name-based closure
unserialization would ship a universal, app-independent gadget in the
engine: name system, feed args from the same payload, done. The
declared-set boundary reduces that to swapping one closure a class already
declares for another.

I won't commit to an RFC that turns every serialized payload into that kind
of gadget, and I don't think we should, either.

That said, I offered to split the RFC and drop the serialize part from the
first one. The reflection API is the piece Symfony needs most: with it,
symfony/cache can export the closures itself for any PSR-6/16 consumer.
What only the engine can provide is provenance, "is this closure declared
by an attribute, and which one". I tried to build it in userland and in an
extension; the extension has to instrument ReflectionAttribute and mirror
private engine structs, and the result is still an approximation.

The format is very complex and deals with a ton of edge cases, e.g.
> handling so many attribute target types. It's mostly hidden from users,
> but not completely (referring to the reflection API).
>
> The implementation is also very large and complex.
>

The target types are the ones the language gives attributes; covering fewer
would leave arbitrary holes ("this closure serializes, that one doesn't,
because it hangs on a constant"). On size: two flag lines in the compiler,
everything else in one file, no new persisted state, no opcache changes.

Overall, I'm sadly not in favor of this RFC.
>

I hope that's not the last word, because the problem is real and only the
engine can address it; referencing closures found in attributes is generic,
not a Symfony quirk. Would the reflection-only subset address your
complexity concern? That would help me decide whether to reshape the
proposal or its wording.

Nicolas

---

## #132010 — Nicolas Grekas — 2026-07-18T10:33:44+00:00

URL: https://externals.io/message/131198#132010 · In reply to: #131939

Dear all,




Le mer. 15 juil. 2026, 17:52, Nicolas Grekas <nicolas.grekas+php@gmail.com>
a écrit :

> Thanks for having a look
>
> Le mer. 15 juil. 2026 à 00:20, Ilija Tovilo <tovilo.ilija@gmail.com> a
> écrit :
>
> > […]
>
> Attributes are one metadata source among several, in systems designed
> around loaders: Doctrine entity metadata, Symfony Serializer mappings,
> Validator constraints, to name a few widely used ones. Loaders read XML,
> YAML, phpdoc in its day, attributes now, and produce a common
> representation, merged and indexed along the way. That derived
> representation is what goes into cache layers or gets dumped as PHP (a
> cache layer too).
>
> So the comparison is not newInstance() vs `unserialize()` for one attribute:
> it is re-running the whole load+merge+index pipeline on every request vs
> one cache fetch. https://github.com/symfony/symfony/issues/63228
> describes how closures in attributes disrupt this: the engine gives no way
> to put them in a cache (via `serialize()` or exported PHP), so affected
> classes fall back to runtime extraction. What you suggest is what is
> implemented today, and it is the measured slow path. This RFC closes the
> gap by letting the already-optimized representation be cached.
>
>
>
> > […]
>
> Concrete pointers to unclear spots would help, I'll happily rework them.
> The short description on the PR may be a better entry point:
> https://github.com/php/php-src/pull/22716. The RFC is long because it
> maps the problem space and the alternatives I explored; that context felt
> necessary to judge the choices.
>
>
>
> > […]
>
> The failure set is narrower than that. References are scoped to their
> declaring element and carry a hash of the closure's code: shifting elements
> around, adding methods, reordering, moving the class in the file, none of
> that invalidates anything. A reference fails when its own closure's code
> changed, when closures were added/removed in its own element, or when the
> element or class was renamed. And when the closure's code changed, failing
> is the correct outcome: the cached metadata was derived from the old code,
> so the entry must be rebuilt, not partially reused.
>
> Also, today's behavior on the comparable event (a renamed class) is
> arguably worse than an exception: __PHP_Incomplete_Class propagates
> silently until first use. Caches cannot return data they fail to restore
> per PSR-6/16; treating a failed unserialize as a miss is how conforming
> implementations behave.
>
>
>
> > […]
>
>
>
> > […]
>
>
> We did add allowed_classes, and we do defer __wakeup()/__unserialize()
> calls until the whole payload is reconstructed, both because some gadget
> surfaces were too risky despite that doc note. Security is not black and
> white; hardening is about bounding consequences when users err. In Symfony
> we also reject gadget chains as non-CVE, and we still fix them as
> hardening. See
> https://github.com/ambionics/phpggc/tree/master/gadgetchains for how
> effective chains are in practice.
>
> The magnitude here is specific. Today a chain needs app-specific gadget
> classes; that is why phpggc is a per-framework catalog. Name-based closure
> unserialization would ship a universal, app-independent gadget in the
> engine: name system, feed args from the same payload, done. The
> declared-set boundary reduces that to swapping one closure a class already
> declares for another.
>
> I won't commit to an RFC that turns every serialized payload into that
> kind of gadget, and I don't think we should, either.
>
> That said, I offered to split the RFC and drop the serialize part from the
> first one. The reflection API is the piece Symfony needs most: with it,
> symfony/cache can export the closures itself for any PSR-6/16 consumer.
> What only the engine can provide is provenance, "is this closure declared
> by an attribute, and which one". I tried to build it in userland and in an
> extension; the extension has to instrument ReflectionAttribute and mirror
> private engine structs, and the result is still an approximation.
>
> The format is very complex and deals with a ton of edge cases, e.g.
> > […]
>
> The target types are the ones the language gives attributes; covering
> fewer would leave arbitrary holes ("this closure serializes, that one
> doesn't, because it hangs on a constant"). On size: two flag lines in the
> compiler, everything else in one file, no new persisted state, no opcache
> changes.
>
> Overall, I'm sadly not in favor of this RFC.
> > […]
>
> I hope that's not the last word, because the problem is real and only the
> engine can address it; referencing closures found in attributes is generic,
> not a Symfony quirk. Would the reflection-only subset address your
> complexity concern? That would help me decide whether to reshape the
> proposal or its wording.
>
> Nicolas
>


After sleeping on this for a few days, I worked on v0.3 of this RFC.

Unlike was I suggested previously, I didn't reduce the proposal to
Reflection only but to the other way: I removed reflection-related changes
and kept only the `serialize()` part.

This makes things simpler and solve the generic case in a better way: while
reflection would have helped the implementation I already built, I'm mostly
interested in providing a solution that doesn't require any dependency. The
problem is generic, so should be the solution.

 `serialize()` is the one feature that most/all cache systems are built on, so
that's the place that needs the improvement.

I also reworded the body of the RFC to hopefully address Ilia's concerns
about it, which others might share.

I'm looking forward to new feedback on this new version.

https://wiki.php.net/rfc/serializable_closures

Cheers,
Nicolas