I have worked with SOAP services from time to time but now i have actually even nicer way to debug them. I still use SOAPUI as its an amazing tool but now i can easily see what is PHP doing :- )
Before, i would simply log everything in raw xml with headers etc. before i sent it out and whenever i got a response back i would log hat too. This is a great way to keep history of what was actually sent and what did we actually get back in case of any investigations etc. It does not impact the performance that much if your volumes of calls are low so you can even enable it temporarily on production servers to see what is there.