I've got this SOAP request structure I must comply in my petition:

<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
 <soap:Body>
  <isValidUser xmlns="http://lab.domain.com/">
   <userId>int</userId>
   <lastName>string</lastName>
  </isValidUser>
 </soap:Body>
</soap:Envelope>

and I wrote in my PHP code

$params = array('userId' => '5555', 'lastName' => 'Smith');
$response = $client->__soapCall("isValidUser", $params);

but I generate this request message:

<SOAP-ENV:Body>
<ns1:isValidUser/>
<param1>Smith</param1>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

Which code must I write in PHP to reproduce the XML request I need?

Thank you very much.