From phpBB Development Wiki
These are use cases phpBB4 should be able to handle.
OpenID, LDAP and local registrations
Users should be able to use their OpenID, login with their ldap account or choose to register locally. Or any combination thereof.
OpenID & external profile source
Users should be able to login using OpenID, their profile information shall be taken from an external source (e.g. a Joomla! installation). The profile needs to stay up to date. Changes should either propogate to the external system or not be allowed.