The fastest and easiest way to install PHP on IIS is using Microsoft’s Web Platform installer. It completely automates setting up IIS, FastCGI and the latest version of PHP from the php.net site. To install it, just click this button:

Also included in the installer are ready packaged popular web applications currently including:
Drupal, DotNetNuke, Photo Gallery, Wiki, SilverStripe CMS, Umbraco and WordPress.
Now we just need Microsoft to add MySQL and we’re sorted!