MantisBT is a free popular web-based bugtracking system (feature list). It is written in the PHP scripting language and works with MySQL, MS SQL, and PostgreSQL databases and a webserver. MantisBT has been installed on Windows, Linux, Mac OS, OS/2, and others. Almost any web browser should be able to function as a client. It is released under the terms of the GNU General Public License (GPL).
MantisBT's codebase dates back to a time when PHP's support for object-oriented programming principles was in its infancy. As of version 1.2.0, the majority of the MantisBT codebase still uses procedural programming principles. Some sections of the MantisBT codebase have been converted to make use of PHP 5's new object model.