$unified['sign'] = self::getSign($unified, $config['key']);

        $responseXml = $this->curlPost('https://api.mch.weixin.qq.com/secapi/pay/refund', self::arrayToXml($unified));

        $unifiedOrder = simplexml_load_string($responseXml, 'SimpleXMLElement', LIBXML_NOCDATA);

        if ($unifiedOrder === false) {

            die('parse xml error');

然后后台显示 parse xml error 这是哪里错了？怎么错的