Zend_Search_Lucene is a php port of the Apache Lucene project, a full-text search engine framework. ZendSearchLucene 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.
I prefer Swish-E myself. It’s what powers this site, and I have a simple PHP interface I built for it.
In a related note, I’m doing some work on an existing ColdFusion site. For all of ColdFusion’s shortcomings, search is not one of them. CF has had integration with a bundled Verity search engine since I can’t remember when — as far back as version 4.5 at least.