How do I input soap method parameter requiring xml parameter string

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PHP

Hello,
I would be grateful if anyone can assist me. I need to get a response from a web service that has a soap request as described below among others:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:quic="http://services.thedomain.com/theservice/">
   <soapenv:Header/>
   <soapenv:Body>
      <quic:GetBillers>
         <!--Optional:-->
         <quic:xmlParams><![CDATA[<SearchCriteria>
<TerminalId>4pbl0001</TerminalId>
</SearchCriteria>]]></quic:xmlParams>
      </quic:GetBillers>
   </soapenv:Body>
</soapenv:Envelope>

I used the code below for another action (which does not need any request fields) of the same web service, and I got the correct response:

 $client = new SoapClient($wsdl,array('trace' => $trace, 'exceptions' => $exceptions));
   $response = $client->GetBillerCategories();
var_dump($response);

However the problem I have is that for the GetBillers request shown at the top, I used the same code (with the addition of arguments) as I did for the successful GetBillerCategories, but could not get an appropriate response. The modified code is shown below:

$xml_array['TerminalId'] = '4pbl0001';
$xml_array['CategoryId'] = 4;
$client = new SoapClient($wsdl,array('trace' => $trace, 'exceptions' => $exceptions));
$response = $client->GetBillers($xml_array);
var_dump($response);

The response I got was:

object(stdClass)#36 (1) { ["GetBillersResult"]=> string(144) "100018464014" }

instead of a long string of names, Ids, products and other details of the billers of CategoryId 4.
What do I need to do to get the full GetBillers response?
Your assistance would be highly appreciated.