You can use SoapClient::_getLastRequest() and SoapClient::_getLastResponse(), then log the output using error_log or another logging platform.
You could then use a tool like SoapUI to manually send requests to the server, which would help in debugging requests.
You need to set 'trace' to true in the options array of the constructor for these methods to be enabled.
http://au1.php.net/manual/en/soapclient.getlastrequest.php http://au1.php.net/manual/en/soapclient.getlastresponse.php