First of all, if you are starting from scratch (i.e. you haven’t installed any version of PHP yet), then there is no need to read the rest of my post – you should read this post by Ruslan Yakushev (Program Manager on the IIS team): Multiple PHP Versions on the Same IIS Server.
However, if you have installed at least one version (via the WPI of Windows Installer, then you can benefit from his post. There's a relatively simple three step process to it - deleting an environment variable, restarting and manually installing the other PHP versions. It's the PHPRC environment variable that determines which version IIS looks for.