HTTP/1.1 200 OK
Proxy-Connection: Keep-Alive
Connection: Keep-Alive
Content-Length: 15872
Via: **** // proxy server name
Expires: 0
Date: Tue, 20 Oct 2009 22:41:37 GMT
Content-Type: application/vnd.ms-excel
Server: Apache/2.2.11 (Unix) DAV/2 mod_ssl/2.2.11 OpenSSL/0.9.8i mod_python/3.3.1 Python/2.5.2 SVN/1.4.6 mod_apreq2-20051231/2.6.0 mod_perl/2.0.4 Perl/v5.10.0
Cache-Control: private
Pragma: no-cache
Last-Modified: Tue, 20 Oct 2009 22:41:37 GMT
Content-Disposition: attachment; filename="myFile.xls"
Vary: Accept-Encoding
Keep-Alive: timeout=5, max=100

// 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 (isset($_SERVER['HTTP_USER_AGENT']) && (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== 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);