Problem with chroot in apache

Hi
I have a query regarding implementation of chroot in
apache.
We have hosted a website in /var/www/html and in this path we have a
directory called forum i.e /var/www/html/forum/ and i want that
whenever a user forum logs in, he should have only acces to this
directory (/var/www/html/forum/) and its files and subfolders. I have
no idea how to implement that.
Any kind of advice regarding this is welcome.

Note: Apache, PHP, MySQL is used for site hosting and PHPBB is used
for forums.

Regards
Saurabh

.