On his blog today, Samisa Abeysinghe points out a library that can be used to trace the SOAP messages going back and forth between your client and server - the WSO2 Web services framework for PHP.
The Web services client class, WSClient has two methods for this, getLastRequest() and getLastResponse(). After calling the request() method of the client instance, you can call any of those methods to gain access to the messages.
Simple example code is included showing how to grab that information from the client object. If you're looking for a way to get the WSF/PHP functionality installed on your system, check out this step-by-step tutorial on using the PECL tool to create and install the packages to your PHP setup.