public static function singleton() {
    if (self::$_instance === null) {
      if (isset($_SESSION['shop'])) {
        self::$_instance = unserialize($_SESSION['shop']);
      } else {
        self::$_instance = new shop();
        self::$_instance->load();

        $_SESSION['shop'] = serialize(self::$_instance);
      }
    }

    self::$_instance->setDefines();
    return self::$_instance;
  }