You're
better using an auth plugin. I'm sure there's already one for
PhpBB login.
There is something out there already but the problem is that it's in the
wrong direction, ie you have to login using mediawiki instead of the other
way round.
Are you sure on it?
An auth plugin fires on login, so it can authenticate the user against
the phpbb password db, and create the mw user if not existing for mediawiki.
In fact,