"Unable to parse URL" exception after SOAP request

Your resulting request is OK (equivalent to expected request from documentation). Problem is there is no endpoint address specified in wsdl http://publicbetawebservices.hotel.de/V2_8/FreeHotelSearchWebService.svc?WSDL. So SoapClient does not know URL where to post the request and you get error Unable to parse URL. You have to specify endpoint URL manually in code. For SoapClient this should work:

$client = new SoapClient('http://publicbetawebservices.hotel.de/V2_8/FreeHotelSearchWebService.svc?WSDL',array("trace"=>1));

Check this post also.