php实现文件下载的一段代码

$file_dir=$totalDirectory;
     $file_name=$filename;
     //echo './db/'.$file_dir.$file_name;
    if (file_exists($file_dir.$file_name)){
          $file=fopen($file_dir.$file_name,'r');
         Header('Content-type:application/octet-stream');
         Header('Accept-Ranges:bytes');
         Header('Content-Disposition:attachment;filename='.$file_name);
         echo fread($file,filesize($file_dir.$file_name));
         fclose($file);
         exit;
        
     }else{
         echo 'file is not exists';
     }