If all you need is a search tool, consider Google Custom Search or FastFind. The first requires no real development, just setting up a Google account and following instructions on how to get Google to index your site, including an advanced option where you use a special script which generate an XML file Google can read and update if you want to control the index better. Google even has an Ajax front end if you wish. The latter is a fully featured back end indexer/front end search box generator complete with templates for customization. I can't supply the link due to a firewall issue here at work, just search for "php fastfind" in Google.
Two simple suggestions. If you want a proprietary search tool, i.e. one that queries your database directly (or via indexing) and is customized to work with that data set (i.e. search by very specific field types) then look into MySQL search engines, here is one . Use it to learn how MySQL does fulltext queries. This is intended to help you get started on a your own, if you opt to code it from scratch.
@NogDogOct 29.2010 — #If you want to write your own search code, it will help if your articles are stored in a MyISAM table type and a "full text" index is defined for the columns you'll be searching on. Then you can use the [url=http://dev.mysql.com/doc/refman/5.1/en/fulltext-search.html]full text search functions[/url] to grab the data.