Papa Google points me to this Zend article with lots of good examples on both the client and server aspects of working with Soap (in particular PHP5's implementation of it). Looks like a good starting point.

If you're somewhat like me, and cringe at the thought of writing up a WSDL by hand, I'd recommend using WSHelper, which uses PHP's reflection classes to dynamically generate a WSDL for you. Definitely a time-saver