Php force Download - downloading corrupted file

Hi I am trying to allow download a demo file which is related to a product. my code is like this

if(isset($_POST['submit'])){     
    $root = $_SERVER['DOCUMENT_ROOT'];
    $path = "/download/";
    $path = $root.$path;
$filename = $demo;
$file = $path.$filename;
header('Content-Type: application/force-download');
header("Content-Transfer-Encoding: Binary"); 
header("Content-Disposition: attachment; filename=\"".basename($file)."\";" );
header("Content-Length: ".filesize($file));
 readfile($file);
    }

I am able to get file name which is associated with the product by

$demo

After user submits his information, download will start automatically. Now this is downloading a non existing file or corrupted file with the proper file name. please help

<?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;
}

?>

As you can see Content type is application/octet-steam meaning file is byte by byte encoded. Also the cache headers are set. Then headers are forcefully sent by ob_clean();flush(); and then the file is read.

The file_exists is there to ensure that given file exists. You should also try not not thrust user input as they could easy write names for your php codes and download EACH file. And with ../ in names of the files, even your documents or system files and so on.