O’Reilly has an excellent tutorial on using the PEAR SOAP module in PHP:
Web services allow you to exchange information over HTTP using XML. When you want to find out the weather forecast for New York City, the current stock price of IBM, or the best-selling DVD according to Amazon.com, you can write a short script to gather that data in a format you can easily manipulate. From a developer’s perspective, it’s as if you’re calling a local function that returns a value.