I have a short answer which I have used in a project. In my answer, I consider that you have visitor IP address.
$ip = "202.142.178.220";
$ipdat = @json_decode(file_get_contents("http://www.geoplugin.net/json.gp?ip=" . $ip));
//get ISO2 country code
if(property_exists($ipdat, 'geoplugin_countryCode')) {
echo $ipdat->geoplugin_countryCode;
}
//get country full name
if(property_exists($ipdat, 'geoplugin_countryName')) {
echo $ipdat->geoplugin_countryName;
}