Firefox works, IE doesn't.

The code I have works fine in firefox, but it seems that in IE it doesn't load when I have the "include" statement under "default". Does anyone know why this might be? Is there a bug in IE?

switch ($screen_res)
{
case "1280x1024":
  echo '1280x1024';
  include("1280x1024.php");
  break;
case "1024x768":
  echo '1024x768';
  include("1024x768.php");
  break;
case "1680x1050":
  echo '1680x1050';
  include("1680x1050.php");
  break;
case "800x600":
  echo '800x600';
  include("800x600.php");
  break;

default:
  echo 'new';
  include("1280x1024.php");
  if(isset($HTTP_COOKIE_VARS["users_resolution"]))
  mail(email@email.com","Screen Resolution","A user with IP address: ". $REMOTE_ADDR . " attempted to view your page with a resolution of " . "$screen_res");
}

PHP:

 

note that the include statement under default is the same as the include statement under 1280x1024.