/** * Main Sensei_Course_Progress Instance * * Ensures only one instance of Sensei_Course_Progress is loaded or can be loaded. * * @since 1.0.0 * @static * @see Sensei_Course_Progress() * @return Main Sensei_Course_Progress instance */ public static function instance($file, $version = '1.0.0') { if (is_null(self::$_instance)) { self::$_instance = new self($file, $version); } return self::$_instance; }