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.
Douma goes into detail about the physical requirements (disk space, processing power) needed for setting up a local search engine, while examining the indexing process and search results list.
Some of the search engines ranked and compared includes iSearch, MnogoSearch, PHPDig and Sphider. Very useful information if you plan to set up a local search engine and happen to be using PHP for your website.