php gets user's real IP

If the user uses a proxy server, the real IP is not in the HTTP_CLIENT_IP header, but needs to be parsed through the http header HTTP_X_FORWARDED_FOR.

The following php function:

function GetIP() { //Get IP
 if ($_SERVER["HTTP_X_FORWARDED_FOR"])
 $ip = $_SERVER["HTTP_X_FORWARDED_FOR"];
 else if ( $_SERVER["HTTP_CLIENT_IP"])
 $ip = $_SERVER["HTTP_CLIENT_IP"];
 else if ($_SERVER["REMOTE_ADDR"])
 $ip = $_SERVER["REMOTE_ADDR"];
 else if (getenv( "HTTP_X_FORWARDED_FOR"))
 $ip = getenv("HTTP_X_FORWARDED_FOR");
 else if (getenv("HTTP_CLIENT_IP"))
 $ip = getenv("HTTP_CLIENT_IP");
 else if (getenv("REMOTE_ADDR"))
 $ip = getenv("REMOTE_ADDR");
 else
 $ip = "Unknown";
 return $ip;
}
?>

php