Make sure you have all the following lines somewhere in your httpd.conf (or in your conf.d/php.conf):

LoadModule php4_module modules/libphp4.so

# Cause the PHP interpreter to handle files with a .php extension.
AddType application/x-httpd-php .php

# Add index.php to the list of files that will be served as directory
DirectoryIndex index.php

Hope that helps.