In php there are some situation where you want to download file Forcefully.For that you have to use php header function. In header function you have to use Content-Type as application/octet-stream and Content-Disposition.

  ob_clean();
  ob_start();
  $filename='private_movie.zip';
  $absoluteFilePath='/public_html/downloads/private_movie.zip'
  header("Pragma: public");     
  header ("Cache-Control: must-revalidate, post-check=0, pre-check=0");
  header("Cache-Control: maxage=1"); 
  header ("Content-Type: application/octet-stream");
  header ("Content-Length: " . filesize($absoluteFilePath));
  header ("Content-Disposition: attachment; filename=$filename");
  readfile($absoluteFilePath);