I made a typo, sorry:
/* phpBB3 Integration */
define('IN_PHPBB', true);
$phpbb_root_path = '/your/absolute/path/to/forum/'; /* As a 'rule' I usually try to define absolute paths */
$phpEx = 'php';
include($phpbb_root_path . 'common.' . $phpEx);
// Start session management
$user->session_begin();
$auth->acl($user->data);
$user->setup();
include('/path/to/class/file.php'); /* Oops, thats what I get for rushing. Include at the bottom */
unset($user);unset($auth);
/* End phpBB3 Integration */