I took an old PC and installed Ubuntu Server on it. There is a very nice installation wizard that asks you what to install. You just check the items you want and it installs them for you. You end up with the same thing as XAMPP plus a Linux server. After the install, you no longer need a keyboard, mouse, or monitor.
Plug it into your switch and you end up with a local testing server server on your network. You also have the benefit of another backup of your files. If you choose to install SAMBA, you can also store files from PCs, Macs, and Linux machines that can be access by any of the other machines on the network.