first you have to install the php package, or compile the source (probably you have
Then in bottom of your /etc/apache/httpd.conf you have to uncomment the line that includes the mod_php.conf file.
Or include this lines if you prefer.
LoadModule php4_module libexec/libphp4.so
AddType application/x-httpd-php .php
It's usefull to include index.php in the DirectoryIndex directive.