Vyřešil jsem to tak:

<?php 

function detect_ie() 
{
  if (isset($_SERVER['HTTP_USER_AGENT']) &&  (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== false))
    return true;
  else
    return false;
}
 
function verze_ie() {  
 $match=preg_match('/MSIE ([0-9]\.[0-9])/',$_SERVER['HTTP_USER_AGENT'],$reg);
 if($match==0)
   return -1;
 else
   return floatval($reg[1]);
}
 
if (detect_ie() && verze_ie() < 9 ) { 
  header('Location: /unsupported.html');
    }
    ?>