I'm running RedHat 7.2, Apache 2.0.44, PHP 4.3.1.
First, if you've compiled PHP as a DSO (compile option: -with-apxs2=/path/to/apache/bin/apxs), make sure you load the module:
LoadModule php4_module modules/libphp4.so
That should be in your httpd.conf file along with:
AddType application/x-httpd-php .php
Then restart the server: apachectl restart
-a9