if (getenv(HTTP_X_FORWARDED_FOR)) {
$pipaddress = getenv(HTTP_X_FORWARDED_FOR);
$ipaddress = getenv(REMOTE_ADDR);
echo "Your Proxy IP address is : ".$pipaddress. " (via $ipaddress) " ;
} else {
$ipaddress = getenv(REMOTE_ADDR);
echo "Your IP address is : $ipaddress";
}
?>
See PHP source code here