yii\mongodb\file\StreamWrapper::register PHP Method

    public static function register($protocol = 'gridfs', $force = false)
    {
        if (in_array($protocol, stream_get_wrappers())) {
            if (!$force) {
                return;
            }
            stream_wrapper_unregister($protocol);
        }
        stream_wrapper_register($protocol, get_called_class(), STREAM_IS_URL);
    }