public static function baseUri($mode = null): string
    {
        switch ($mode) {
            case Wechat::MODE_DEV:
                self::getInstance()->baseUri = 'https://api.mch.weixin.qq.com/sandboxnew/';
                break;

            case Wechat::MODE_HK:
                self::getInstance()->baseUri = 'https://apihk.mch.weixin.qq.com/';
                break;

            default:
                break;
        }

        return self::getInstance()->baseUri;
    }