Cross Directory Sessions With phpbb

I am trying to set up a forum for a website I built a while back. If possible I would also like to integrate the user account system/sessions from phpbb with the rest of my site.

 

So if i go from mysite.com/forum

to mysite.com/other_directory

I don't want users to be logged out.

 

There doesn't seem to be any official documentation or tutorials on this, but I know its doable since thats how most sites seem to work. If anyone can point me in the right direction I'd appreciate it. Thanks.