It's a bit silly, but it's worth writing it down, as there will be a day you'll have to configure php on IIS in a Preproduction via VPN with a really pushing customer and only few minutes available... and somehow you WILL forget this.
Php.ini by default has the short code set to off, so if you configure everything and test it out with <? phpinfo() ?> you will a blank screen with the code in the source, which can also indicate that there is something wrong in the configuration and that the script doesn't get sent to the php processor. It's not, it's correct! Just test out with <?php phpinfo(); ?> or turn on the short codes!