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Date: Wed, 12 Jun 2024 17:06:58 +0200
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Subject: [PHP-DEV] Property hooks, and &get-only write behavior
To: PHP internals <internals@lists.php.net>
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From: tovilo.ilija@gmail.com (Ilija Tovilo)

Hi everyone

While reviewing the property hooks patch, Bob found a discrepancy in
the implementation and the specification. There is an unfortunate
mistake in one of the examples that suggests specific by-reference
behavior that we did not intend to support.

Specifically, from https://wiki.php.net/rfc/property-hooks#references
(you'll have to scroll down a bit):

```php
class C {
    public string $a {
        &get {
            $b = $this->a;
            return $b;
        }
    }
}
$c = new C();
$c->a = 'beep';
// $c is unchanged.
```

This example implies that the assignment `$c->a = 'beep';` will invoke
`&get` to write to the contents of the reference. However, this is not
the case. Direct assignments will, in the absence of `set`, fall back
to the builtin write operation for backed properties, or throw for
virtual properties.

Instead of `$c->a = 'beep';`, the example _should_ have said:

```php
$ref = &$c->a;
$ref = 'beep';
```

Here, `$ref = &$c->a;` _will_ call `&get`. What the example intended
to show is that the reference returned by `&get` is detached from
`$c->a` itself.

That said, I did give this idea some thought. I think it would not
have been unreasonable to automatically call `&get` and assign to the
returned reference when there is a `&get` hook but _no_ `set` hook.
However, when trying to implement this, I encountered some technical
issues that make this currently unfeasible. So, we'd like to move
forward with the original semantics: If there is no `set` hook, then
the assignment will trigger a builtin write for backed properties, or
an error for virtual properties.

If you wish to achieve the behavior described above (using `&get` to
write to the property), you can implement `set` to perform the write
to the reference yourself.

I hope that these semantics are acceptable for everyone.

Ilija