From a resource perspective PHP is much easier on the system then mod_perl/mysql. PHP was designed to work with mysql and allows a lot of cool features in that arena. I am not sure why its less resource intensive, but I know it is based on performance testing we've done using wwwthreads and other php message boards.
There was also an article in a linux magazine recently about memory leaks using constant connections in mod_perl and mysql. We've been experiencing this for a while and never could figure out what the problem is. We have very active boards (20,000 members).
Other notes, if you are using a site with PHP this opens the doors to fully integrate wwwthreads into everything else on your site. That is the biggest benefit to us. Perl cannot print out php, so all of our headers/footers and php code had to be hacked to work with wwwthreads in the past. Now it can be seamless.
Thanks again for porting!
Will