The CallOptions header replaces the setClientId method in the old toolkit. The first argument to CallOptions is a Client and the second is a namespace prefix for managed packages. Both are optional.

Sample Call

<? php
define("SOAP_CLIENT_BASEDIR", "../../soapclient");
require_once (SOAP_CLIENT_BASEDIR.'/SforcePartnerClient.php');

try {
  $mySforceConnection = new SforcePartnerClient();
  $mySoapClient = $mySforceConnection->createConnection(SOAP_CLIENT_BASEDIR.'/partner.wsdl.xml');
  $callOptionsHeader = new CallOptions('YourClientId', 'aNamespace');
  $mySforceConnection->setCallOptions($callOptionsHeader);
  $loginResult = $mySforceConnection->login($username, $password);
  //print_r($mySforceConnection->getLastRequest());
  //print_r($loginResult);
} catch (Exception $e) {
  echo $e->faultstring;
}
?>