<a href='download.php?f=$filename'>Файл</a>

 
if (!empty($_GET['f']))
 {
  $file = "uploads/".$_GET['f'];
  header ("Content-Type: application/octet-stream");
  header ("Accept-Ranges: bytes");
  header ("Content-Length: ".filesize($file));  
  header ("Content-Disposition: attachment; filename=". basename($file));  
  readfile($file);
 }

 if (!empty($_GET['f'])){
    $file = "uploads/".$_GET['f'];
    if (ob_get_level()) { ob_end_clean(); }
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename=' . basename($file));
    header('Content-Transfer-Encoding: binary');
    header('Expires: 0');
    header('Cache-Control: must-revalidate');
    header('Pragma: public');
    header('Content-Length: ' . filesize($file));
    readfile($file);
    exit;
 }