SOAP：错误的请求

I am trying to access the web service. The below provided code gives the exception message: "Bad Request". Is there any way to see what exactly is wrong in my request? How to debug this code in order to see which parameters in $p are not correct?

try
{
    $webServices = $client->queryTest($p);
} 
catch (Exception $e) {
    print $e->getMessage();
}