On the "System Architect" site there's a recent post showing you how to integrate PHP and Solr, the searching tool from the Apache project.
So why do you need a search engine, is database not enough? If you create a small website it might not matter. With medium or big size applications it's often wiser to go for a search engine. Saying that, even a small websites can benefit from Solr if you desire a high level of relevance in search results.
Their example involves an ecommerce website and a search for a term (iPhones) and how difficult it could be to match against the possible multiple variations on the models. Solr makes this kind of searching easier. He shows you how to get a Solr instance all set up and configured as well as the PHP PECL extension from here. A sample PHP script is also included showing connecting to Solr, inserting a new document and searching for a simple query of "hello".