NuSOAP is a solid library for PHP that enables the easy creation of SOAP servers and clients. We used it in development for a simple test client and for the web service itself. The test client and a tweaked version of NuSOAP that remove conflicts with PHP 5 class names are uploaded here.
The home page for NuSOAP is:
An excellent NuSOAP tutorial that was very helpful for learning how to implement SOAP web services and use NuSOAP to do that was written by Scott Nichol:
Notice the four links at the bottom of that page go to a tutorial set he has created that is pretty comprehensive. For our QuickBooks development and testing we found it easiest not to use WSDL in our test client. No big deal... it doesn't limit you at all in your development.
Scott Nichol's tutorials were uploaded against the day when that link might become broken. This isn't a foreseeable event, but we want to make sure his work is preserved. All credit goes to Scott Nichol!