Try sending your parameters in an array.
$client=new SoapClient('http://test.XXXX.com/DFPortal/Portal.asmx?wsdl');
$params['customer'] = $value;
$client->ExecuteRequestWithLogin($params);
which would be the same is saying
$client=new SoapClient('http://test.XXXX.com/DFPortal/Portal.asmx?wsdl');
$res = $client->ExecuteRequestWithLogin( array("customer"=>$value) );
Additionally, remember that if you have a response it will come back as an stdClass and not an array.