From a pointer from the Zend Developer Zone, there's a link to this case study of the creation of a search engine surrounding the Zend_Search component of the Zend Framework.

We have implemented Zend_Search into our content management system "Click and Change" and we would like to share our experiences with the developers' community. For this purpose, we decided to publish the complete source code and it's documentation.

They give a great overview of the application, from some of the interesting things they came across during development to some of the feedback/questions they've already recieved about the project. They have the source code for each of the files in the search engine, including small changes made after the fact. You can also download the entire project in one zipped up file.