Here’s a good article from Zend showing you how to create your own search engine using Zend_Search_Lucene (part of the Zend Framework).

“One of the features that sets the Zend Framework apart from the others is the inclusion of a decent search module. Zend_Search_Lucene is a php port of the Apache Lucene project, a full-text search engine framework. Zend_Search_Lucene promises a simple way to add search functionality to an application without requiring additional php extensions or even a database.

Zend_Search_Lucene overcomes the usual limitations of relational databases with features such as fast indexing, ranked result sets, a powerful but simple query syntax, and the ability to index multiple fields. Better still, a Zend_Search_Lucene index can live happily alongside your relational database to provide fast searching but without duplicating the effort of storing all of your data twice. In this tutorial, I’ll show you how to use Zend_Search_Lucene to index and search some RSS feeds.”
Link: Roll Your Own Search Engine with Zend_Search_Lucene