You'll want to write a bridge that makes use of the session handling functionality of the forum software. I assume you are actually sharing whatever the user table is that correct?
Effectively, the sessions need to match something that phpbb understands. I don't use phpbb and don't know their structure, but they likely have a separate handling file just for its sessions. You want to make use of that in your home page to bridge the forum software to your software.
header('HTTP/1.1 420 Enhance Your Calm');