redirect domain based on country

There's not really any advantage I can think of to run this logic server-side.
I would just use javascript/jquery and then you just use an API to resolve the visitor country code by IP and redirect to the corresponding URL :

Code:

$.getJSON("http://api.db-ip.com/v2/free/self").then(function(addrInfo) {
  if (addrInfo.countryCode== "US") {
      document.location = "http://us.example.com";
  } else if (...) {
      // handle other cases
  } else {
      document.location = "http://example.com";
  }
});

Should you still want a server-side api call, here is a PHP library for the demonstrated api: https://github.com/dbip/api-client

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