Okay, did a quick setup on my computer, and it works great.
Just follow the instructions on the extension page
. (It goes in the extensions directory of mediawiki, nothing to phpBB is changed.)
Just make sure that "$wgAuth_Config['UseWikiGroup']" is false and "$wgAuth_Config['UseExtDatabase']" is true. (I assume that the wiki and phpbb databases are separate for the latter. If so make sure to un-comment the MySQL settings and set them accordingly.)
Also make sure that "$wgAuth_Config['UserTB']", "$wgAuth_Config['GroupsTB']" and "$wgAuth_Config['User_GroupTB']" are properly set, took me 5 minutes before I realized that they were set to "phpbb3_---" instead of "phpbb_---"
Oh, and that "$wgAuth_Config['PathToPHPBB']" is set to '../forums'
You can also implement "additional security"
to prevent banned users from logging in to the wiki.