?? its not somthing you would search the registry for, you must have a running server, and then download and set up PHP, i say setup because PHP is not somthing that installed persay.
to see if it is installed. create a PHP file like this
[PHP]<?php
phpinfo(); //will display all of the phpinfo if php is installed else will print out as seen by the server
?>[/PHP]
save it is. info.php
open up your web server and go to
http://www.localhost/info.php
if it works and you see info then your lucky and have correctly installed PHP if not, you have to install it.