How to implement forced download file function in php

public function down()

{
  $id = $this->_get('id');
  $M = M("downloads");
  $data=$M->where("id=$id and status=1")->find();
  !$data && exit;
  $filename = iconv('UTF-8','GBK',$data['filename']);
  $savename = $data['savename']; 
  $myfile = $data[url] ? $data[url] : 'Uploads/file/'.$savename;
  if(file_exists($myfile)){
  　　$M->where("id=$id")->setInc('downloads');
  　　$file = @ fopen($myfile, "r");
  　　header("Content-type: application/octet-stream");
  　　header("Content-Disposition: attachment; filename=" .$filename );
  　　while (!feof($file)) {
    echo fread($file, 50000);
  　　}
  　　fclose($file);
  　　exit;
  }else{
  　　echo '文件不存在！';

  }
}