PHPBB Forum needs to work with site\'s user DB - repost

Hello, I have a website where users are kept in record in a table named- tbl_users and now I installed a forum using same database but with prefix of tables phpbb_. I want when someone is also registered at the site to be able to use the forum with same username and password. Please, send me PM with you plan to do it- PHP login modification, auth_db tweak or something else.