Open Source PHP Search Engine – Yioop!
Yioop! is an open source search engine built with PHP and uses MySQL or SQLite for storing the data.
It can be configured to crawl any page it finds by following links or limited to any given number of URLs and used as an in-site search engine (like Google Custom Search).
A standard setup can crawl and index millions of pages per day and this number can be increased by running more crawlers (distributed setup is possible as well).
There is support for indexing many file types including HTML, DOC, PNG, JPG, GIF, XML, sitemaps, RSS and much more.
Also, the system can be controlled completely from a web-based administration interface.