From the WS02 Oxygen Tank website, there's a new tutorial focused on the use of the PHP SOAP extension in an application:
This article by Samisa Abeysinghe explains the SOAP extension that can be useful in providing and consuming Web services using PHP. He details the classes of extension, two implementation models of Web services, and also lists a 'Hello World with SOAP Extension'.
True to the summary, they start with an introduction to what SOAP is an how the communication with it is performed (via a handy graphic). They move from there on to the different parts of the SOAP connection in PHP - SoapServer, SoapFault, SoapHeader, SoapParams, and the SoapVal classes.
They explain what WSDL is and how to can help and move quickly into their "Hello World" example - a simple SOAP server that returns, what else, "Hello world" to the calling script (providing both the client and server sides of the equation). They include the raw response so you can get an idea of what kind of message is being passed back and forth.