I've recently just tangled with PHP and Web Services. Something that I want to point out is the use of SOAP verses XML-RPC. What I can say is that XML-RPC came very easy to use.
Why? ...Keith Devens XML-RPC Library for PHP is by far the best to work with setting up Web Services using XML-RPC.
Some words from Mr. Keith...
"The main benefit of using this XML-RPC library over others is that it uses native PHP datatypes wherever possible and doesn't force you into OO. I'd tried other XML-RPC libraries, and they make you use objects in complex ways just to build your XML-RPC message. With this library, you can build your message using nothing but native PHP datatypes. Since PHP is a high level scripting language, it has most of the type information available to you that you need to build an XML-RPC message. That's called "reflection" :) The library is also fairly small (<500 lines)."
I mean I tried using SOAP, but as for a beginner at working with Web Services it was just ridiculous. And I could imagine it can be sometimes difficult for experienced Web Service programmers too.
Don't be afraid of dropping the SOAP...
Here is an example Web Service that I set up using Keith Devens XML-RPC Library for PHP. (Look at the service.php(server) and the wgo_api.php(client) files).
Beginners and experienced Web Service programmers would appreciate this XML-RPC library.
Be sure to download Keith Devens Library.
-Rashaud