The above comments are spot on. I actually use a PHP CMS for my customers (CMS Made Simple) as the Perl ones don't really cut it. That one is simple and good and does the job. If you want more power, consider Joomla. The code in phpbb2 was so insane the last time I looked that I wouldn't recommend using it if you want to customise the code.

You can find a useful summary of CMSes and their features at Wikipedia List_of_content_management_systems

Cheers, Peter