On DevShed a new series has started (a six-part one) with This new tutorial posted today talking about the use of the Yahoo! web services in a PHP5 application.
It's not my intention to provide you here with a fully-featured guide on how to use Yahoo!'s Web services with PHP 5, because the best place to go is simply the aforementioned Developer Network. Instead, I'll offer to you a bunch of code samples that hopefully will assist you in the development of PHP 5 applications that will utilize some of these useful web services.
You'll need to get an Application ID to work with before you get started (go here to sign up, you'll also need a Yahoo! ID), but the rest of the code is provided. They make a search request to the Yahoo! backend for "madonna" and show the XML formatting of the results.
They also include examples of querying two other services offered by the company's backend API - video search and an image search.