Ruslan Yakushev has written up a new post detailing how you can set up an IIS server (traditionally only able to handle one PHP version instance at a time) to use multiple versions depending on your needs.
This post describes how to configure IIS to support multiple versions of PHP on the same server. This kind of setup is useful in development environments where it is necessary to test an application with different PHP versions. Also, it is often used in production environments, where many PHP applications are hosted on the same server and some of them have dependency on a particular PHP version.
He gives instructions for three IIS versions - 5.1, 6.0 and 7.0 and later. The trick to it is in the FastCGI configuration file. His technique lets you set up alias to the different PHP installations (as many as you want) and a "Types" section where it can easily be swapped out. With IIS 7.0 and later, some of this can be done through the IIS control panel.