Cómo saber la IP real del usuario con PHP

<?php
function SacarIP() {
  if($_SERVER) {
    if($_SERVER["HTTP_X_FORWARDED_FOR"]) {
      $realip = $_SERVER["HTTP_X_FORWARDED_FOR"];
    } elseif ($_SERVER["HTTP_CLIENT_IP"]) {
      $realip = $_SERVER["HTTP_CLIENT_IP"];
    } else {
      $realip = $_SERVER["REMOTE_ADDR"];
    }
  } else {
    if(getenv("HTTP_X_FORWARDED_FOR")) {
      $realip = getenv("HTTP_X_FORWARDED_FOR");
    } elseif(getenv("HTTP_CLIENT_IP")) {
      $realip = getenv("HTTP_CLIENT_IP");
    } else {
      $realip = getenv("REMOTE_ADDR");
    }
  }
  return $realip;
}
echo SacarIP();
?>