Please don’t test with unsupported versions of PHP. Even if the testing is automated, it is wasted effort. If a customer of yours is on an unsupported version of PHP, then it is your duty as a developer to tell them, “Sorry, I’d love to help you, but you are running outdated, unsupported and possibly insecure version of PHP. I can help you by getting you up to a more modern version of PHP, then my library/ software will work great for you too!”
Oh, and if they do ask for your help to upgrade their system to a higher version of PHP, your experience using a Linux VM locally will come in real handy. At that point, those devs using Windows only to develop on will be shrugging their shoulders. 
Scott