Form Chris Shiflett's blog, WSDL Despite PHP 5 by Christian Wenz:

We are working a lot with web services these days. One of the things I like best, from a technical perspective, is the WSDL standard. WSDL is used to provide information about a SOAP web service: operation, data types, messages, location, and the like. Using WSDL, consuming a web service is very easy. Most technologies support an implementation of the proxy pattern and can automatically create a local proxy object from a WSDL description.

This entry looks into PHP5 SOAP extension, NuSOAP as well as WSF/PHP.