In order to successfully install the PHP manager for IIS 8, you need the .NET 3.5 framework installed and enabled, and for some reason, that‘s not part of the setup, which does download the .NET 2.0 framework.
So, just add the .NET 3.5 framework manually (you can do so through the "add features" wizard), and the your PHP manager should install properly. And don‘t forget to apply the security patches from Windows update for the new Frameworks before sending this box into production.