php, file download

One issue I have with the following code is you have no control over the output stream, your letting PHP handle it without knowing exactly what is going on within the background:

What you should do is set up an output system that you can control and replicated accros servers.

For example:

if (file_exists($file))
{
    if (FALSE!== ($handler = fopen($file, 'r')))
    {
        header('Content-Description: File Transfer');
        header('Content-Type: application/octet-stream');
        header('Content-Disposition: attachment; filename=".basename($file));
        header("Content-Transfer-Encoding: chunked'); //changed to chunked
        header('Expires: 0');
        header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
        header('Pragma: public');
        //header('Content-Length: ' . filesize($file)); //Remove

        //Send the content in chunks
        while(false !== ($chunk = fread($handler,4096)))
        {
            echo $chunk;
        }
    }
    exit;
}
echo "<h1>Content error</h1><p>The file does not exist!</p>";

This is only basic but give it a go!

Also read my reply here: file_get_contents => PHP Fatal error: Allowed memory exhausted