phpBB

phpBB is a free, open-source forum software platform written in PHP, offering hierarchical categories and subforums, user groups, full-text search, private messaging, and a flat (non-threaded) message structure familiar to millions of longtime forum users. First launched in 2000, it supports multiple database engines including MySQL, MariaDB, PostgreSQL, and SQLite, and can be extended through a large ecosystem of community-built extensions, styles, and language packs available in its official Customisation Database. Licensed under the GPL, phpBB continues active development, with a version 4.0 alpha recently released to modernize its long-standing foundation while preserving the classic forum experience communities have relied on for over two decades.