Get real IP address if user has transportent proxy

Here it is:
<?PHP

function getip()
{
if (getenv("HTTP_CLIENT_IP") && strcasecmp(getenv("HTTP_CLIENT_IP"),"unknown"))
$ip = getenv("HTTP_CLIENT_IP");

elseif (getenv("HTTP_X_FORWARDED_FOR") && strcasecmp(getenv("HTTP_X_FORWARDED_FOR"), "unknown"))
$ip = getenv("HTTP_X_FORWARDED_FOR");

elseif (getenv("REMOTE_ADDR") && strcasecmp(getenv("REMOTE_ADDR"), "unknown"))
$ip = getenv("REMOTE_ADDR");

elseif (!empty($_SERVER['REMOTE_ADDR']) && strcasecmp($_SERVER['REMOTE_ADDR'], "unknown"))
$ip = $_SERVER['REMOTE_ADDR'];

else
$ip = "unknown";

return($ip);
}
?>


source:
http://www.phpinform.com/2009/06/27/get-real-ip-address-if-user-has-transportent-proxy/