//字典排序拼接字符串

	function getWxPaySignature($arr){

	    ksort($arr);

	    $str = '';

	    foreach ($arr as $k=>$v){

	        if($k != "sign" && $v != "" && !is_array($v)){

                $str .= $k . "=" . $v . "&";

            }

	    }

	    $str = trim($str,'&');

	    return $str;

	}

	//微信回调验证

	function weCallbackCheck($xml){

	    $signArr = (array)$xml;

	    $sign = $signArr['sign'];

	    $stringA = getWxPaySignature($signArr);//数组字典排序拼接字符串

	    $stringSignTemp = $stringA.'&key=xxx';//设置的key

	    $signValue = md5($stringSignTemp);

	    $signValue = strtoupper($signValue);

	    if($signValue == $sign){

	        return 'success';//支付成功

	    }else{

	        return 'failure';

	    }

	}

        //使用方法

        $xmlstr = file_get_contents("php://input");

		$xml=simplexml_load_string($xmlstr, 'SimpleXMLElement', LIBXML_NOCDATA);

		if(weCallbackCheck($xml)=="success"){

                      //todo

                }