"php: force file download and ie, yet again" Code Answer

This will check for versions of IE and set headers accordingly.

// assume you have a full path to file stored in $filename
if (!is_file($filename)) {
  die('The file appears to be invalid.');
}

$filepath = str_replace('\', '/', realpath($filename));
$filesize = filesize($filepath);
$filename = substr(strrchr('/'.$filepath, '/'), 1);
$extension = strtolower(substr(strrchr($filepath, '.'), 1));

// use this unless you want to find the mime type based on extension
$mime = array('application/octet-stream');

header('Content-Type: '.$mime);
header('Content-Disposition: attachment; filename="'.$filename.'"');
header('Content-Transfer-Encoding: binary');
header('Content-Length: '.sprintf('%d', $filesize));
header('Expires: 0');

// check for IE only headers
if (preg_match('~MSIE|Internet Explorer~i', $_SERVER['HTTP_USER_AGENT']) || (strpos($_SERVER['HTTP_USER_AGENT'], 'Trident/7.0; rv:11.0') !== false)) {
  header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
  header('Pragma: public');
} else {
  header('Pragma: no-cache');
}

$handle = fopen($filepath, 'rb');
fpassthru($handle);
fclose($handle);