I just did a fresh install of Apache as well as the php_mod module and the server wasn't up and running right away. I got a 403 forbidden error when I went to the root web site. After a while I discovered that the ebuild had created a hdtocs directory in the /home/httpd instead of the htdocs directory that it should have. I'm sure this is just a spelling mistake, easy fix.
I just merged apache and php and a clean system and it worked fine.... Make sure you don't have any stray config files in /etc for apache.