SOAP-Client Fail

I do a SOAP-Client call like:

$client = new SoapClient(null, array(
'location' => WEBSERVICE_BILDVERARBEITUNG_URI_IMAGEPROCESSING."index.php",
'uri' => WEBSERVICE_BILDVERARBEITUNG_URI_IMAGEPROCESSING,
'style' => SOAP_RPC,
'encoding' => 'UTF8',
'trace' => 1));

$retval = $client->doImageProcessing(WEBSERVICE_BILDVERARBEITUNG_AUTH_KEY, trim($xml));

The Constant WEBSERVICE_BILDVERARBEITUNG_AUTH_KEY is a String but the SOAP-Server did not recive this information so I get a Error.

Does anybody know an issue for this problem.