The unknown use of unknown dynamic property within the `WP_List_Table` property magic methods is now deprecated. A descriptive deprecation notice is provided to alert developers to declare the property on the child class extending `WP_List_Table`.
Changes in this commit:
* Adds a deprecation notice to the `__get()`, `__set()`, `__isset()`, `__unset()` magic methods, i.e. to alert and inform developers when attempting to get/set/isset/unset a dynamic property.
* Fixes `__get()` to explicitly returns `null` when attempting to get a dynamic property.
* Removes returning the value when setting a declared property, as (a) unnecessary and (b) `__set()` should return `void` [https://www.php.net/manual/en/language.oop5.overloading.php#object.set per the PHP handbook].
* Adds unit tests for happy and unhappy paths.
For backward compatibility, no changes are made to the internal declared properties listed in `$compat_fields` and accessed through the magic methods.
For example:
A child class uses a property named `$data` that is not declared / defined as a property on the child class. When getting its value, e.g. `$list_table->data`, the `WP_List_Table::__get()` magic method is invoked, the following deprecation notice thrown, and `null` returned:
>The property `data` is not defined. Setting a dynamic (undefined) property is deprecated since version 6.4.0! Instead, define the property on the class.
=== Why not remove the magic methods, remove the `$compat_fields` property, and restore the properties `public`?
tl;dr Backward compatibility.
Several plugins, one of which has over 5M installs, add a property to the `$compat_fields` array. Removing the property would cause an `Undefined property` `Warning` (PHP 8) | `Notice` (PHP 7) to be thrown. Removing the associated code would change the functionality.
=== Why not limit the deprecation for PHP versions >= 8.2?
tl;dr original design intent and inform
The magic methods and `$compat_fields` property were added for one purpose: to continue providing external access to internal properties declared on `WP_List_Table`. They were not intended to be used for dynamic properties.
Deprecating that unintended usage both alerts developers a change is needed in their child class and informs them what to change.
References:
* Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.
* A [https://www.youtube.com/live/vDZWepDQQVE?feature=share&t=10097 live open public working session] where these changes were discussed and agreed to.
* [https://wiki.php.net/rfc/deprecate_dynamic_properties PHP RFC: Deprecate dynamic properties.]
Related to #14579, #22234, #30891.
Follow-up to [15491], [28493], [28521], [28524], [31146].
Props antonvlasenko, jrf, markjaquith, hellofromTonya, SergeyBiryukov, desrosj, peterwilsoncc, audrasjb, costdev, oglekler, jeffpaul.
Fixes#58896.
See #56034.
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