Code Modernization: Deprecate dynamic properties in WP_User_Query magic methods.

The unknown use of unknown dynamic property within the `WP_User_Query` 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_User_Query`.

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.
* Fixes `__set()` by removing the value return after 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].
* Fixes `__isset()` to return `false` if not in the `$compat_fields`, as `isset()` and `__isset()` should always return `bool`:
   * [https://www.php.net/manual/en/language.oop5.overloading.php#object.isset `__isset()` in the PHP manual] 
   * [https://www.php.net/manual/en/function.isset.php `isset()` in the PHP manual] 
* 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 `$data` that is not declared as a property on the child class. When getting its value, e.g. `$user_query->data`, the `WP_User_Query::__get()` magic method is invoked, the following deprecation notice thrown, and `null` returned:

>The property `data` is not declared. Setting a dynamic property is deprecated since version 6.4.0! Instead, declare 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.

If a plugin adds a property to `$compat_fields` array, then sites using that plugin would experience (a) an `Undefined property` `Warning` (PHP 8) | `Notice` (PHP 7) and (b) a possible change in behavior.

=== 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_User_Query`. 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, #27881, #30891.

Follow-up to [15491], [28528], [31144].

Props antonvlasenko, rajinsharwar, jrf, markjaquith, hellofromTonya, SergeyBiryukov, desrosj, peterwilsoncc, audrasjb, costdev, oglekler, jeffpaul.
Fixes #58897.
See #56034.
Built from https://develop.svn.wordpress.org/trunk@56353


git-svn-id: http://core.svn.wordpress.org/trunk@55865 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-user-query.php

@@ -1109,6 +1109,7 @@ class WP_User_Query {
 	 * Makes private properties readable for backward compatibility.
 	 *
 	 * @since 4.0.0
+	 * @since 6.4.0 Getting a dynamic property is deprecated.
 	 *
 	 * @param string $name Property to get.
 	 * @return mixed Property.
@@ -1117,27 +1118,42 @@ class WP_User_Query {
 		if ( in_array( $name, $this->compat_fields, true ) ) {
 			return $this->$name;
 		}
+
+		trigger_error(
+			"The property `{$name}` is not declared. Getting a dynamic property is " .
+			'deprecated since version 6.4.0! Instead, declare the property on the class.',
+			E_USER_DEPRECATED
+		);
+		return null;
 	}
 
 	/**
 	 * Makes private properties settable for backward compatibility.
 	 *
 	 * @since 4.0.0
+	 * @since 6.4.0 Setting a dynamic property is deprecated.
 	 *
 	 * @param string $name  Property to check if set.
 	 * @param mixed  $value Property value.
-	 * @return mixed Newly-set property.
 	 */
 	public function __set( $name, $value ) {
 		if ( in_array( $name, $this->compat_fields, true ) ) {
-			return $this->$name = $value;
+			$this->$name = $value;
+			return;
 		}
+
+		trigger_error(
+			"The property `{$name}` is not declared. Setting a dynamic property is " .
+			'deprecated since version 6.4.0! Instead, declare the property on the class.',
+			E_USER_DEPRECATED
+		);
 	}
 
 	/**
 	 * Makes private properties checkable for backward compatibility.
 	 *
 	 * @since 4.0.0
+	 * @since 6.4.0 Checking a dynamic property is deprecated.
 	 *
 	 * @param string $name Property to check if set.
 	 * @return bool Whether the property is set.
@@ -1146,19 +1162,34 @@ class WP_User_Query {
 		if ( in_array( $name, $this->compat_fields, true ) ) {
 			return isset( $this->$name );
 		}
+
+		trigger_error(
+			"The property `{$name}` is not declared. Checking `isset()` on a dynamic property " .
+			'is deprecated since version 6.4.0! Instead, declare the property on the class.',
+			E_USER_DEPRECATED
+		);
+		return false;
 	}
 
 	/**
 	 * Makes private properties un-settable for backward compatibility.
 	 *
 	 * @since 4.0.0
+	 * @since 6.4.0 Unsetting a dynamic property is deprecated.
 	 *
 	 * @param string $name Property to unset.
 	 */
 	public function __unset( $name ) {
 		if ( in_array( $name, $this->compat_fields, true ) ) {
 			unset( $this->$name );
+			return;
 		}
+
+		trigger_error(
+			"A property `{$name}` is not declared. Unsetting a dynamic property is " .
+			'deprecated since version 6.4.0! Instead, declare the property on the class.',
+			E_USER_DEPRECATED
+		);
 	}
 
 	/**

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-alpha-56352';
+$wp_version = '6.4-alpha-56353';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.