Class Extension

Le 08/07/2026 à 14:07, Michael Morris a écrit :

On Wed, Jul 8, 2026 at 1:08 AM Holly Schilling
holly.a.schilling@outlook.com wrote:

Following the discussion in Discord yesterday, I started working
on the feasibility of implementing class extensions. I believe
I’ve proven out the concept and viability of it in a 2-phase
approach using an extension syntax similar to that of Swift.

Before I go into much more detail, what is the general interest in
adding this to the language?

We have inheritance (extends keyword, parent / child classes)
We have traits (an implementation of multiple classes taken from Ruby)

Looking at Swift's extension syntax I fail to see anything it adds not
covered by the above.

I think that Swift's extension system goes the other way round: the
class itself doesn't declare its extensions, but they are declared from
userland.

It's similar to how Rust's traits system can allow extending other types
(though there's kind of a safeguard to avoid extending nonsense) , you
can check it there: https://doc.rust-lang.org/book/ch10-02-traits.html

PHP with inheritance goes this way:

• Declare Class A
    - Class A internally defines which class it inherits from
    - Class A internally defines which traits it uses
• Everything is now defined for Class A and its structure is finite as
  of the end of its declaration

With an "extension" system, it might go this way:

• Declare Class A
    - Class A internally defines which class it inherits from
    - Class A internally defines which traits it uses
• Class A is defined, but extensions can update it somehow.
• An extension is later declared for Class A: it can customize the inner
  structure of Class A, like new traits or implemented interfaces
• Maybe (depending on how the RFC goes) other extensions can be declared
  later, in order to customize other things.

Conceptually, I'm not against it, but just like Michael said: I can't
find use-cases that would be better than what we actually have.
Extending a class "from the outside", whatever the way to do it, implies
that a new safeguard has to be added. We could use the "final" keyword,
but "final" is not at all the same thing as "no extension allowed": a
class could be declared as not-final while still disallowing
extensibility from the outside. That's the case when you have a
non-final class but some of its methods are "final". Disallowing
extensibility would require an entirely new keyword for that, and it
adds another burden to code maintainers, especially if extensibility is
opt-out.

So, IMO, such RFC should contain this:

• Thorough examples of the actual problems it solves for package
  maintainers and application developers
• Clear views on how extensibility impacts the current ecosystem (I
  guess it shouldn't have any impact on Composer and the autoloading
  system, because if it's similar to Swift, an extension should just
  behave like a class in terms of autoload, but I might have not thought
  about it completely)
• Extensibility must be either opt-in by design, or opt-out by design,
  but the decision must be made after having analysed the impact on
  the ecosystem. I can't imagine extensibility being opt-out and people
  starting to wildly override frameworks core classes. (yeah, I'm talking
  about some Laravel nerds that tend to bend and break fast (pun intended))
• Extension points must be defined on both the extender and the extended
  structures. Like, for example, making sure it's impossible to extend
  something that's "final" by default, but making a non-final structure
  being non-extensible should also be possible. That's part of the concept
  of encapsulation in general, but it also respects the Open/Close
  principle (from SOLID) and several other programming practices that make
  sure we don't do bullcrap in all codebases. And trust me, PHP is already
  full of bullcrap, so adding more isn't a good idea at all. PHP hasn't
  become "more and more strict" over time for nothing... and still, that
  strictness is opt-in (by the eyes of PHP, not from framework
  maintainers, which can be seriously opinionated).

Hope this helps :)

Following the discussion in Discord yesterday, I started working on the feasibility of implementing class extensions. I believe I’ve proven out the concept and viability of it in a 2-phase approach using an extension syntax similar to that of Swift.

Before I go into much more detail, what is the general interest in adding this to the language?

We have inheritance (extends keyword, parent / child classes)
We have traits (an implementation of multiple classes taken from Ruby)

Looking at Swift's extension syntax I fail to see anything it adds not covered by the above.

Traits and extensions approach a similar problem from the opposite direction. Where a class uses a trait to add functionality, instead, an extension declares that it has added functionality to a class (or interface).

Besides allowing one avenue for default implementations for interface methods, extensions would also allow for polyfills for class methods and properties that can be hooked. For example, PHP 8.5 adds the (sadly not documented yet!) Dom\Element::getElementsByClassName() and Dom\Element::insertAdjacentHTML() methods. If PHP 8.4 already had extensions, then extensions would have made it possible for a userspace library to provide an implementation for those methods on PHP 8.4.

In the hypothetical case where we have method calls on scalar types, extensions could also be used as a means of adding methods to those types (either in userspace, or internally or in php extensions).

-John