Hello,
I had a similar problem, and it took me hours to figure out. When I installed PHP 4.3.4, it inserted this line in my httpd.conf:
AddType application/x-http-php .php
Instead of processing my php files, the source was sent to the browser. After extensive googling, I discovered that the line should be "application/x-httpd-php". So, try it without the 'd'. That must be correct for some systems, or it wouldn't be the default.
(I was installing as a module on Apache 2.0.48 under YellowDog Linux.)