PHP SOAP using only one SoapParam passed

Why does this:

$a = 'val1';
$b = 'val2';

$client->__soapCal开发者_开发知识库l('Request', array(   new SoapParam($a, "Par1"),
                                        new SoapParam($b, "Par2") ));

return only this?:

<?xml version="1.0" encoding="UTF-8"?>
   <SOAP-ENV:Envelope xmlns:SOAP-ENV="http://domain.wsdl" xmlns:ns1="https://domain.wsdl">
     <SOAP-ENV:Body>
      <ns1:Request/>
         <Par1>val2</Par1>
      </SOAP-ENV:Body>
   </SOAP-ENV:Envelope>

I expected 2 pars and Par1/Par2 inside Request.