Downloaded file (using download.php) from server won't open. Why?

Question

I am a beginner. I've searched everywhere, but I don't know why after downloading a file from the server through the client (Win 7 Firefox), I am unable to open the file. I've tried a PNG file and an MP4 file. The download completes, but the files don't open. Here's my script;

$dl_file = $_GET['val']; //Verified the full path and the file name gets passed here
$basename = basename($dl_file);
$ext = pathinfo($dl_file, PATHINFO_EXTENSION);
$length   = sprintf("%u", filesize($dl_file));

    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename='.$basename.'');
//manually tried '.$basename.'.PNG' - DID NOT work. How to pass $ext here?
        header('Content-Transfer-Encoding: binary');
        header('Connection: Keep-Alive');
        header('Expires: 0');
        header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
        header('Pragma: public');
        header('Content-Length: ' . $length);

        set_time_limit(0);
        readfile($dl_file);

I can't imagine why the downloaded file won't open. Is it corrupt? Please throw more light on this. Thank you in advance.

Answer 1

after many tries, I found that adding the 2 lines of code shown below (ob_clean, and flush) (and/or adding precheck, postcheck parameters) - worked on all browsers.Thanks.

if (file_exists($dl_file)) {
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename="'.basename($dl_file).'"' );
    header('Expires: 0');
    header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
    header('Pragma: public');
    header('Content-Length: ' . filesize($dl_file));
    ob_clean();
    flush();
    readfile($dl_file);
    exit;
}