This Authentication backend only works against phpBB version 3.x, also known as phpBB3.
It won't work against phpBB2 due to a different password hashing algorithm.

To Enable phpBB Authentication you will need to do the following three steps:

To enable phpBB Authentication within the Wiki goto the Admin page (Login section -icon of a door and arrow) and in dropdown box select phpBB:

Make sure that Registration is turned off:

User registration is handled in the phpBB software.

Goto the phpBB tab:

You need to provide the database hostname, user, password, database name and table prefix (if any).

Below is a table of the options and what they do.

When the above is completed the wiki is ready to use phpBB as an authentication source. You will now need to ensure that phpBB is setup correctly.
You can do that easily by logging in using a phpBB username/password, and try to log in with a non-existing user, and/or wrong password.