Example of using mod_xsendfile to send a file as a download.

mod_xsendfile_example.php

<?php
/**
 * In your Apache conf file, set the following to enable XSendfile:
 * 
  <Directory "/">
    # This should be handled by the XSendFile module, but a bug in 0.11.1 prevented it from being set properly.
    EnableSendfile on
  
    XSendFile on

    # Absolute path to where your download files are stored. No trailing slash!
    XSendFilePath "C:\inetpub\apacheroot\mysite\download_files"
  </Directory>
 */

function join_paths($paths)
{
  if (!is_array($paths)) $paths = func_get_args();
  foreach ($paths as $i => $part) {
    $part = trim(preg_replace('|[\\\\/]+|', DIRECTORY_SEPARATOR, trim($part)) , DIRECTORY_SEPARATOR);
    if (!strlen($part)) unset($paths[$i]);
    else $paths[$i] = $part;
  }
  return join(DIRECTORY_SEPARATOR, $paths);
}
function can_haz_xsendfile()
{
  // This will return false if PHP is not loaded as a module (i.e. uses cgi)
  return in_array('mod_xsendfile', apache_get_modules());
}
function send_download_package_file($base_path, $file_path)
{
  $realpath = join_paths(DOWNLOAD_PACKAGE_BASE_ROOT, $base_path, $file_path);
  if (file_exists($realpath)) {
    // Fetching File
    $mtime = ($mtime = filemtime($realpath)) ? $mtime : gmtime();
    $size = intval(sprintf("%u", filesize($realpath)));
    header("Content-type: application/force-download");
    header('Content-Type: application/octet-stream');
    if (strstr($_SERVER["HTTP_USER_AGENT"], "MSIE") != false) {
      header("Content-Disposition: attachment; filename=" . urlencode(basename($file_path)) . '; modification-date="' . date('r', $mtime) . '";');
    } else {
      header("Content-Disposition: attachment; filename=\"" . basename($file_path) . '"; modification-date="' . date('r', $mtime) . '";');
    }
    if (can_haz_xsendfile()) {
      // Sending file via mod_xsendfile
      header("X-Sendfile: " . join_paths($base_path, $file_path));
    } else {
      // Sending file directly via script
      if (intval($size + 1) > return_bytes(ini_get('memory_limit')) && intval($size * 1.5) <= 1073741824) { //Not higher than 1GB
        // Setting memory limit
        ini_set('memory_limit', intval($size * 1.5));
      }
      @apache_setenv('no-gzip', 1);
      @ini_set('zlib.output_compression', 0);
      header("Content-Length: " . $size);
      // Set the time limit based on an average D/L speed of 50kb/sec
      set_time_limit(min(7200, // No more than 120 minutes (this is really bad, but...)
      ($size > 0) ? intval($size / 51200) + 60 // 1 minute more than what it should take to D/L at 50kb/sec
       : 1 // Minimum of 1 second in case size is found to be 0
      ));
      $chunksize = 1 * (1024 * 1024); // how many megabytes to read at a time
      if ($size > $chunksize) {
        // Chunking file for download
        $handle = fopen($realpath, 'rb');
        $buffer = '';
        while (!feof($handle)) {
          $buffer = fread($handle, $chunksize);
          echo $buffer;
          ob_flush();
          flush();
        }
        fclose($handle);
      } else {
        // Streaming whole file for download
        readfile($realpath);
      }
    }
    return true;
  } else {
    // File not found! Throw error here...
  }
  return false;
}

// Absolute path, same as what you set for the XSendFilePath directive
define('DOWNLOAD_PACKAGE_BASE_ROOT', 'C:\inetpub\apacheroot\mysite\download_files');

// Any intermediate path between XSendFilePath and your file. It should have neither a leading nor trailing slash
define('DOWNLOAD_PACKAGE_DIR', 'pdfs');

if (send_download_package_file(DOWNLOAD_PACKAGE_DIR, 'some_big_file.pdf')) {
  // Exit successfully. We could just let the script exit
  // normally at the bottom of the page, but then blank lines
  // after the close of the script code would potentially cause
  // problems after the file download.
  exit;
}
// else, raise error...
?>

return_bytes.php

<?php
function return_bytes($val)
{
  $val = trim($val);
  $last = strtolower($val[strlen($val) - 1]);
  switch ($last) {
  // The 'G' modifier is available since PHP 5.1.0  
  case 'g':
    $val*= 1024;
  case 'm':
    $val*= 1024;
  case 'k':
    $val*= 1024;
  }
  return $val;
}
?>