I've found a php class to integrate a user login system (such as MX Kollektion) with phpBB forum. I have some problems to know exactly how to do that integration. Below is the explanation given in the download (in attachment).
/*
* These examples show you how to integrate the phpBB
* login in and authentication system with your website.
*
* Since we quite like our login system and it's proven
* itself to be very extensible, we don't want to replace
* but we do want to have a universal login system for
* both our website *and* the forum.
*
* To take full advantage of this PHPBB_Login class you'll
* need to modify your own login system to include a call
* to the relevant login or logout methods.
*
* This way, you can handle all of the website login as normal,
* and also log the user into phpBB in the same step.
*
* This system is reliant on the website username being exactly
* the same as the phpBB username. To insure this, I recommend
* disabling the ability to change usernames from within the
* phpBB admin control panel.
*
* Distributed under the LGPL license:
* http://www.gnu.org/licenses/lgpl.html
*
* Duncan Gough
* 3rdSense.com
*
* Home http://www.suttree.com
* Work http://www.3rdsense.com
* Play! http://www.playaholics.com
*/
/* Example 1: Logging in */
session_start();
/* First, login the user using your own login system, for example; */
$user = new User();
// username and password are implied here,
// they will most likely be form variables
$user->login( $username, $password );
// Then login the user to the forum
$phpBB = new PHPBB_Login();
$phpbb->login( $user->id );
/* Example 2: Logging out */
session_id();
$user = new User();
/* First, logout the user from the forum */
$phpBB = new PHPBB_Login();
$phpbb->logout( session_id(), $user->id) ;
/* Then logout the user from your own login system */
$user->logout( $user->id );
