Sphider is a lightweight web spider and search engine written in PHP, using MySQL as it's back end database. It is a great tool for adding search functionality to your web site or building your custom search engine. Sphider is small, easy to set up and modify, and is used in thousands of websites across the world.
Sphider supports all standard search options, but also includes a plethora of advanced features such as word autocompletion, spelling suggestions etc. The sophisticated adminstration interface makes administering the system easy.