How to get the client IP address using PHP

To get the client IP address, PHP provides a server variable $_SERVER. The $_SERVER can contain keys with IP.

function getClientIP(){
    foreach (['HTTP_CLIENT_IP','HTTP_X_FORWARDED_FOR','HTTP_X_FORWARDED','HTTP_X_CLUSTER_CLIENT_IP','HTTP_FORWARDED_FOR','HTTP_FORWARDED','REMOTE_ADDR'] as $key){
        if (isset($_SERVER[$key])){
            foreach (explode(',', $_SERVER[$key]) as $ip){
                $ip = trim($ip);

                if (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) !== false) {
                    return $ip;
                }
            }
        }
    }
    return null;
}

echo getClientIP();