Saber la IP del usuario con PHP

Hola, esta es un función para saber la IP de usuario, hay que ver que no este detrás de un proxy ya que si es el caso no tendremos la ip ‘real’.

/**
 * Saber la ip del usuario
 */
function getRealIP() {
  $ipaddress = '';
  if (!empty($_SERVER['HTTP_CLIENT_IP']))
    $ipaddress = $_SERVER['HTTP_CLIENT_IP'];
  else if(!empty($_SERVER['HTTP_X_FORWARDED_FOR']))
    $ipaddress = $_SERVER['HTTP_X_FORWARDED_FOR'];
  else if(!empty($_SERVER['HTTP_X_FORWARDED']))
    $ipaddress = $_SERVER['HTTP_X_FORWARDED'];
  else if(!empty($_SERVER['HTTP_FORWARDED_FOR']))
    $ipaddress = $_SERVER['HTTP_FORWARDED_FOR'];
  else if(!empty($_SERVER['HTTP_FORWARDED']))
    $ipaddress = $_SERVER['HTTP_FORWARDED'];
  else if(!empty($_SERVER['REMOTE_ADDR']))
    $ipaddress = $_SERVER['REMOTE_ADDR'];
  else
    $ipaddress = 'UNKNOWN';

  return $ipaddress;
}