Why is my downloaded file is always damaged or corrupted?

Question

I have a very weird problem with my download script

it basically

1.gets a file id with "GET" method

2.gets the name and location

Answer 1

    if (file_exists($file)) {
set_time_limit(0);
        header('Connection: Keep-Alive');
    header('Content-Description: File Transfer');
            header('Content-Type: application/octet-stream');
            header('Content-Disposition: attachment; filename="'.basename($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);


}