Like many popular modern forum systems, phpBB started out as a UBB clone, in mid 2000. phpBB 1.0.0 was released by December 2000, and the 1.x codebase was improved and enlarged until the release of version 1.4.4 in November 2001. The next release, 2.0.0, was developed from scratch and released in April 2002, and was distributed with a new default design, which, while considered overused by some, has influenced the designs of several competitors.
The current development branch (2.1.x, to be released as 2.2.0 when production quality) is building upon the new codebase to add the functionality omitted from previous versions, and optimise the code above the level that was allowed by the strict compatibility rules placed on 2.0.x.