We can't help you - we have no idea which PC the device is connected to, or how the "measuring device" communicates with the connected PC. I can assume it's via serial data, but that's not guaranteed. So start by looking at the device manufacturers website, and find out how the PC needs to talk with it or listen to it. Get that working first!
If it's the Server, then which of the Clients do you want to talk to? Why that one? You would need to think carefully about exactly what you are trying to achieve, and then start here: PHP serial data - Google Search