Phorum project is a web based message board written in PHP. Phorum is designed with high-availability and visitor ease of use in mind.
Features such as mailing list integration, easy customization and simple installation make Phorum a powerful add-in to any website.
As of Phorum 5.0, PHP 4.2.x or higher is required. Some people have used it with earlier versions, but we can not support you on it. There were some key changes in that version that are beneficial to Phorum.
You can get PHP from the PHP site or if you are leasing web space, your provider may already have it installed. As database there is currently only MySQL supported while it should be possible to port the database layer to other databases. MySQL is available from MySQL AB. It is OpenSource. Just download, decompress, configure and go.
Another great thing about Phorum is that it is OpenSource software and is licensed under the Phorum License (Very similar to the PHP License and Apache License). This means it is free! But you need to be sure and read the license before using it.