function download_document($filename,$path="",$mimetype="application/octet-stream")
{
 header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
 header("Content-Disposition: attachment; filename = $filename");
 header("Content-Length: " . filesize($pathto . $filename));
 header("Content-Type: $mimetype");
 echo file_get_contents($pathto . $filename);
}

<?php
$file = "1.txt";// 文件的真实地址（支持url,不过不建议用url）
if (file_exists($file)) {
  header('Content-Description: File Transfer');
  header('Content-Type: application/octet-stream');
  header('Content-Disposition: attachment; filename='.basename($file));
  header('Content-Transfer-Encoding: binary');
  header('Expires: 0');
  header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
  header('Pragma: public');
  header('Content-Length: ' . filesize($file));
  ob_clean();
  flush();
  readfile($file);
  exit;
}
?>