To create complex SOAP Headers, you can do something like this:

Required SOAP Header:

<soap:Header>
    <RequestorCredentials xmlns="http://namespace.example.com/">
      <Token>string</Token>
      <Version>string</Version>
      <MerchantID>string</MerchantID>
      <UserCredentials>
        <UserID>string</UserID>
        <Password>string</Password>
      </UserCredentials>
    </RequestorCredentials>
</soap:Header>

Corresponding PHP code:

<?php

$ns = 'http://namespace.example.com/'; //Namespace of the WS.

//Body of the Soap Header.
$headerbody = array('Token' => $someToken,
                    'Version' => $someVersion,
                    'MerchantID'=>$someMerchantId,
                      'UserCredentials'=>array('UserID'=>$UserID,
                                             'Password'=>$Pwd));

//Create Soap Header.        
$header = new SOAPHeader($ns, 'RequestorCredentials', $headerbody);        
        
//set the Headers of Soap Client.
$soap_client->__setSoapHeaders($header);

?>

You cannot add an additional header. If you want two headers, and one already exists, first delete it with $client->__setSoapHeaders(NULL). Then issue $client->__setSoapHeaders($headers) where $headers is an array of soapHeader() objects.

With multiple SOAP headers, when using SoapVar for creation of SoapHeader the PHP code just terminates (command terminated). I am not sure if that is a bug. 

Without the SOAPVar the code worked fine for me

There are different way to creart SoapHeader I was using SoapVar and the code was not working. I am still a novice with this SOAP stuff.

Tried using normal strings and it worked fine. SoapHeader can take SoapVar or string as the third argument.

my code:

<?php
// first soap header.
$var = new SoapVar($header, XSD_ANYXML);
$soapHeader = new SoapHeader(NAME_SPACE, "Security", $var);
// second soap header.
$var2 = new SoapVar($header2, XSD_ANYXML);
$soapHeader2 = new SoapHeader(DIFF_NAME_SPACE, "ID", $var2);

$client = new SoapClient($wsdl, array("location" => $location));

$headers = array();
$headers[] = $soapHeader;
$headers[] = $soapHeader2;

// Here my code was just terminating.
$client->__setSoapHeaders($headers);
?>