A Web Spider (also commonly referred to as a Web Crawler) is a program which runs indefinitely and visits links on the web for the purpose of generating information about the underlying link-structure of the web. 
Sphider is a lightweight web spider and search engine written in PHP, using MySQL as its 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 - PHP Search Engine Spider: http://www.sphider.eu/
- Mini Bots PHP Class: http://www.barattalo.it/mini-bots-php-class/
- wikipedia: Web Crawler
- My Web Spider: http://php4fun.blogspot.com/2007/11/my-web-spider.html
- Finding What People Want -- Experiences with the WebCrawler (PhD thesis that produced WebCrawler.com): http://www.thinkpink.com/bp/WebCrawler/WWW94.html