PHP调用阿里云API问题

require "ali/TopSdk.php";


    $c = new AliyunClient();
    $c->accessKeyId = "P733dtKLIK******";
    $c->accessKeySecret = "92HKHALX6nzhT43U2********";
    $c->serverUrl="ecs.aliyuncs.com/";
    $req = new Ecs20140526DescribeInstanceAttributeRequest();
    $req->setInstanceId("<InstanceId>");




    try{
        $resp = $c->execute($req);
        if(!isset($resp->Code)){


            //echo ($resp->RequestId);
            echo $resp;
            //print_r($resp);
        }else{


            $code = $resp->Code;
            $message = $resp->Message;
            echo "$message";
        }




    }catch(Exception $e){
        echo "错误请求";


    }




一直提示 The request signature does not conform to Aliyun standards.
格式哪里出问题了