On the Web Development Blog, there's a quick tutorial they've created for replacing a Google search with a Yahoo one with the help of the Zend Framework and it's Service_Yahoo_Websearch component.

During the Zend PHP conference earlier this year, I got in touch with the Zend framework and learned that there is already a class to obtain results from the Yahoo search engine. The class has features to search the Yahoo API for web sites, image, news and the local search results.

They walk through the installation of the library (not the framework, though - you'll need that already set up), grabbing the API key from Yahoo and different applications of performing the search.