code: public static function instance($refresh = false)
      {
          $className = get_called_class();
          if ($refresh || !isset(self::$_instances[$className])) {
              self::$_instances[$className] = Yii::createObject($className);
          }
          return self::$_instances[$className];
      }
docs: Returns static class instance, which can be used to obtain meta information.
  @param bool $refresh whether to re-create static instance even, if it is already cached.
  @return static class instance.