Can I validate customer country in PHP?

Is there some way I could get country some time before payment and after purchase start so I could validate with PHP in system I use? Could I have it connected to a front end so I could get one input from client side Javascript? I could do custom country check before transaction start so really just can I validate country with PHP based on what is in system with me and reject if wrong? I guess this is possible but could I call your API from a back end like PHP?