Michael Douma from PHP Dev Center has come up with a list of PHP-friendly search engines that will allow you to easily integrate customizable search functions on your server while controlling the indexing process and search results.
If you want your visitors to have “voilá!” moments, consider incorporating an internal search engine into your web site. Search tools not only make your information easily accessible, but they also increase the time visitors spend on your site. An internal search engine may be a necessity if your site has more than 100 pages of content, if it is deeply hierarchical, or if its architecture is weak.
PHP, Search Engine, Roundup