Restore $wpdb->use_mysqli for backward compatibility

src/wp-includes/class-wpdb.php

@@ -690,6 +690,16 @@ class wpdb {
 	 */
 	private $allow_unsafe_unquoted_parameters = true;
 
+	/**
+	 * Whether to use mysqli over mysql. Default false.
+	 *
+	 * @since 3.9.0
+	 * @since CP-2.0.0 Defaults to true for backward compatibility.
+	 *
+	 * @var bool
+	 */
+	private $use_mysqli = true;
+
 	/**
 	 * Whether we've managed to successfully connect at some point.
 	 *

tests/phpunit/tests/db.php

@@ -2469,4 +2469,14 @@ public function parse_db_host_data_provider() {
 			),
 		);
 	}
+
+	/**
+	 * The wpdb->use_mysqli is true (for now), purely for backwards compatibility reasons.
+	 */
+	public function test_mysqli_is_set() {
+		global $wpdb;
+
+		$this->assertObjectHasAttribute( 'use_mysqli', $wpdb );
+		$this->assertTrue( $wpdb->use_mysqli );
+	}
 }