Sure that's pretty easy I think... you can even let your user log in on either site. That is just my assumption of course. I did an integration with phpbb before, but it has been a while since. However, I still have the barebones of the script I wrote to integrate little application with the phpbb.
I am looking at a general log in function on the other section of your site, so that they don't have to really be in the forum logging in , and go to the other section of the site to just get validated through the prevailing session from the forum. How about if we do it like this..
On the other section of your site --> we create a log in script -> Upon submit of the form -> user gets validated on your phpbb log in sytem -> if a valid user, the user is then re-directed to the section of your site, where the login attempts was originatin from.
You can then modify the script as you like, but first I want you to see how the integration really works to have the complete view of what is really taking place. After this, you can definitely do whatever it pleases you.
NOTICES and ASSUMPTIONS: I am assuming here that your forum directory is located at YourDOMainDotCom/forum .. Of course you are free to modify the codes for your needs.
STEP ONE : copy codes below and save as other_page.php . This file …