Database: Reinstate wpdb::$use_mysqli property.

Partial revert of [56475] to reinstate the private `wpdb::$use_mysqli` property and set to its default to true. This private property was / is accessible through the magic methods. Though Core's usage of this property was removed by [56475], plugins are using the property. Reinstating it resolves the BC break.

Follow up to [56475].

Reviewed by jorbin.
Merges [57089] to the 6.4 branch.

Props jason_the_adams, joemcgill, johnbillion, johnjamesjacoby, jrf, rajinsharwar, renehermi.
Fixes #59846.

git-svn-id: https://develop.svn.wordpress.org/branches/6.4@57090 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wpdb.php

@@ -690,6 +690,21 @@ class wpdb {
 	 */
 	private $allow_unsafe_unquoted_parameters = true;
 
+	/**
+	 * Whether to use the mysqli extension over mysql. This is no longer used as the mysql
+	 * extension is no longer supported.
+	 *
+	 * Default true.
+	 *
+	 * @since 3.9.0
+	 * @since 6.4.0 This property was removed.
+	 * @since 6.4.1 This property was reinstated and its default value was changed to true.
+	 *              The property is no longer used in core but may be accessed externally.
+	 *
+	 * @var bool
+	 */
+	private $use_mysqli = true;
+
 	/**
 	 * Whether we've managed to successfully connect at some point.
 	 *

tests/phpunit/tests/db.php

@@ -2469,4 +2469,17 @@ class Tests_DB extends WP_UnitTestCase {
 			),
 		);
 	}
+
+	/**
+	 * This private property is no longer used but needs to be retained as it can be
+	 * accessed externally due to the `__get()` magic method.
+	 *
+	 * @ticket 59118
+	 * @ticket 59846
+	 */
+	public function test_use_mysqli_property_access() {
+		global $wpdb;
+
+		$this->assertTrue( $wpdb->use_mysqli );
+	}
 }