From Netmag.co.uk comes a new tutorial looking at working with SOAP messages and responses with PHP:

This issue, we're going to look at my favourite member of the .net trinity: 'Develop'. When working with Google, we had to use Soap and WSDL, two of the simplest methods of defining a strict API for others to build on. We already used the excellent SoapClient class, but there's an equally excellent SoapServer class that also works with WSDL, and we can combine them to share our content with the world, all thanks to PHP.

They start with a look at a sample WSDL document, briefly explaining what the parts are. Following that there's the simple PHP code allows the built-in PHP SOAP functionality (in PHP5) to create a SOAP server that will output/use that WSDL file when clients connect and a simple client that uses the file to define what it can do.