function change($current_banner)
  {
  $number_of_banners = 5;
  if ($current_banner < $number_of_banners)
    {
      $current_banner++;
    }
  else
    {
      $current_banner = 0;
    }
  return $current_banner;
  }

function get_filename($current_banner)
  {
  select $current_banner
  {
     case 1:
       $filename = "thebannername.jpg";

...

  }
}

//Display the banner

//Get filename
$filename = get_filename($current_banner)
//Use filename in HTML
echo "<img src=\"" . $filename . "\">";
//Go to next banner
$current_banner = change($current_banner);