This simple function allows you to detect a country code and city of your site visitors, using the www.geoplugin.net service. You may use this data to add default values in registration form or use it for any other purposes, like blocking access to your site from special regions.

getLocationInfoByIp.php

<?php

# Reference http://www.apphp.com/index.php?snippet=php-get-country-by-ip

function getLocationInfoByIp(){
    $client  = @$_SERVER['HTTP_CLIENT_IP'];
    $forward = @$_SERVER['HTTP_X_FORWARDED_FOR'];
    $remote  = @$_SERVER['REMOTE_ADDR'];
    $result  = array('country'=>'', 'city'=>'');
    if(filter_var($client, FILTER_VALIDATE_IP)){
        $ip = $client;
    }elseif(filter_var($forward, FILTER_VALIDATE_IP)){
        $ip = $forward;
    }else{
        $ip = $remote;
    }
    $ip_data = @json_decode(file_get_contents("http://www.geoplugin.net/json.gp?ip=".$ip));    
    if($ip_data && $ip_data->geoplugin_countryName != null){
        $result['country'] = $ip_data->geoplugin_countryCode;
        $result['city'] = $ip_data->geoplugin_city;
    }
    return $result['city'];
}
?>