How can I allow a user to download a file which is stored outside of the webroot?

Question

I am developing a system which allows registered users (who could be anybody) to upload files. I've block mime-types etc. to attempt to restrict the files to .doc, .docx and .pdf types, but for additional security, they are uploaded to a folder outside the webroot.

Other users can then choose to download the files. How do I allow them to do that? Obviously I can't just put in a link to the file, as it's outside the webroot. I'm not sure how to reach the file though! I presume I can use the php file functions to get to the file, but how do I then 'serve it up' to the user who has requested it?

What security implications might all of this have?

Thanks.

Answer 1

You need a PHP script that does the following:

1. Set the content-type header correctly (depending on what the user is downloading)
2. Set the content-length header correctly (depending on the file size)
3. Open the file for reading (you can use fopen)
4. Read the file and output its content to the output stream
5. Done

You can also use readfile function to do basically the same. Here's an example from PHP's site:

<?php
$file = 'monkey.gif';

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;
}
?>