Zend Framework has one great module called Zend Search Lucene, which I'm going to try for a new project :) :
"Zend_Search_Lucene is a general purpose text search engine written entirely in PHP 5. Since it stores its index on the filesystem and does not require a database server, it can add search capabilities to almost any PHP-driven website. Zend_Search_Lucene supports the following features:
Ranked searching - best results returned first
Many powerful query types: phrase queries, wildcard queries, proximity queries, range queries and more 
Search by specific field (e.g., title, author, contents)"