Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.
There are a number of ways to mitigate this:
* If it is an accidental typo for a declared property: fix the typo.
* For known properties: declare them on the class.
* For unknown properties: add the magic `__get()`, `__set()`, et al. methods to the class or let the class extend `stdClass` which has highly optimized versions of these magic methods built in.
* For unknown ''use'' of dynamic properties, the `#[AllowDynamicProperties]` attribute can be added to the class. The attribute will automatically be inherited by child classes.
Trac ticket #56034 is open to investigate and handle the third and fourth type of situations, however it has become clear this will need more time and will not be ready in time for WP 6.1.
To reduce “noise” in the meantime, both in the error logs of WP users moving onto PHP 8.2, in the test run logs of WP itself, in test runs of plugins and themes, as well as to prevent duplicate tickets from being opened for the same issue, this commit adds the `#[AllowDynamicProperties]` attribute to all “parent” classes in WP.
The logic used for this commit is as follows:
* If a class already has the attribute: no action needed.
* If a class does not `extend`: add the attribute.
* If a class does `extend`:
- If it extends `stdClass`: no action needed (as `stdClass` supports dynamic properties).
- If it extends a PHP native class: add the attribute.
- If it extends a class from one of WP's external dependencies: add the attribute.
* In all other cases: no action — the attribute should not be needed as child classes inherit from the parent.
Whether or not a class contains magic methods has not been taken into account, as a review of the currently existing magic methods has shown that those are generally not sturdy enough and often even set dynamic properties (which they should not). See the [https://www.youtube.com/watch?v=vDZWepDQQVE live stream from August 16, 2022] for more details.
This commit only affects classes in the `src` directory of WordPress core.
* Tests should not get this attribute, but should be fixed to not use dynamic properties instead. Patches for this are already being committed under ticket #56033.
* While a number bundled themes (2014, 2019, 2020, 2021) contain classes, they are not a part of this commit and may be updated separately.
Reference: [https://wiki.php.net/rfc/deprecate_dynamic_properties PHP RFC: Deprecate dynamic properties].
Follow-up to [53922].
Props jrf, hellofromTonya, markjaquith, peterwilsoncc, costdev, knutsp, aristath.
See #56513, #56034.
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Sergey Biryukov
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