$postStr = $GLOBALS['HTTP_RAW_POST_DATA'];

 $resStr = simplexml_load_string($postStr, 'SimpleXMLElement', LIBXML_NOCDATA);

if ($resStr->return_code == 'SUCCESS') {
     $return_info['return_code'] = 'SUCCESS';
     $return_info['return_msg']  = 'OK';
     echo $this->data_to_xml($return_info);  //通知微信支付成功
}

    /**
     * 输出xml字符
     * @param   $params     参数名称
     * return   string      返回组装的xml
     **/
    public function data_to_xml( $params ){
        if(!is_array($params)|| count($params) <= 0)
        {
            return false;
        }
        $xml = "<xml>";
        foreach ($params as $key=>$val)
        {
            if (is_numeric($val)){
                $xml.="<".$key.">".$val."</".$key.">";
            }else{
                $xml.="<".$key."><![CDATA[".$val."]]></".$key.">";
            }
        }
        $xml.="</xml>";
        return $xml;
    }