From phpBB Development Wiki
Obtaining phpBB Source Code
There are several versions to choose from.
Latest stable version of phpBB can be found on the Download page.
When a new version is close to being finished, release candidates are announced on phpbb-qa mailing list.
Current development versions are maintained in Git. Development for all future 3.0.x and later releases is managed with git.
Development branch for current stable version (Olympus, 3.0.x as of this writing) is mirrored in Subversion repository.
Working with phpBB Source Code Checkout
When developing, uncomment the DEBUG and (only in 3.0.x) DEBUG_EXTRA define statements in config.php. Doing so provides additional error reporting, provides errors present in language files, and bypasses the check for the presence of install directory.
define('DEBUG', true); define('DEBUG_EXTRA', true);
You should also enable the "Recompile stale style components" option in the Administration Control Panel (General > Sever Configuration > Load Settings > General options), so your template changes are always displayed and you don't need to refresh the cache after every change.