I tried php5.0, phorum3 and II6.0. Tough luck. I couldn't get past the login page after setting up everything . I even tried the hack. Didn't work. On the other hand, I went back to Php4 and everything started working fine.
I guess phorum3 is not compatible with php5. However this doesn't mean anything is wrong with phorum3. Infact php5 has lot of problems. PHPBB won't work either. So my advice. Stay away from php5.
Chandresh