The IBM developerWorks blog has a new tutorial posted today about using the combination of PHP and Ajax to create a simple chat application (with a database backend).

Learn to build a chat system into your Web application with Asynchronous JavaScript + XML (Ajax) and PHP. Your customers can talk to you and to each other about the content of the site without having to download or install any special instant-messaging software.

Complete code is included (even the database table schemas) to create the simple application. They make the four pages to get it working and call that part of it good. The second part of the article, though, builds on this base and adds a fix for performance issues caused by constant polling of the database by the ajax request.