php强制下载文件函数

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！';  }}