search (here and google) fpr phpbb on mysql5.
i know that there is a mod that will fix phpbb so it can be used on php5, but as of now, it can not do it out of the box.
backup your db and files before you do anything.
absolutely no guarantee.
if you do what i advise and it blows in your face, all you'll hear from me is: "ah... sorry, i guess"