A very simple discussion board program written in PHP. It doesn't use any SQL database.

A simple, easy to use message board system. It supports single or multiple forums, user registration and anonymous posting, and forum administrators. ccForum is written in PHP and is based on a MySQL database. [Open source, GPL]

Customizable PHP bulletin board software that supports different SQL backends [Open source, GPL]

PHP/MySQL forum developed by Umakant Patil that includes group permissions, IP banning, and anti-spam measures.

An open-source, web-based, threaded forum system written in PHP, supporting nested posts and forums.

Forum software written in PHP with a MySQL backend.

Free forum written in PHP. It has also a guestbook, a download area, a photo gallery and a news publisher feature. It uses MySql as database.