With 10.5, Apple switched to Apache2, and that includes PHP5. The default configuration file for Apache2 does not have PHP5 set to "on," so, one has to edit the configuration file. That configuration file is no longer kept in the same directory that it was in under 10.4, so one has to follow a new path to get to the configuration file to edit it.
The new path is
/private/etc/apache2/
and the file to be edited is
httpd.conf
(which I would copy and save somewhere safe).
Because httpd.conf is owned by root, one will have to copy them to another directory where one has write permissions or edit them using sudo-prefaced commands.
There's more info in an Apple forum discussion at
http://discussions.apple.com/thread.jspa?threadID=1187457
However (should I shout "HOWEVER?"), although I've done this, I'm not completely through the maze yet. I now have a server that recognizes and executes PHP pages, but it's choking on WordPress php scripts.
I'm still searching for more information in these fora about resolving that issue.