How to implement automatic segmented download of files in PHP (code example)

function download($file_dir,$file_name)
     //参数说明：
     //file_dir:文件所在目录
     //file_name:文件名
{
     $file_dir = chop($file_dir);//去掉路径中多余的空格
     //得出要下载的文件的路径
if($file_dir != '')
{
     $file_path = $file_dir;
     if(substr($file_dir,strlen($file_dir)-1,strlen($file_dir)) != '/')
           $file_path .= '/';
           $file_path .= $file_name;
}
else
     $file_path = $file_name;
      //判断要下载的文件是否存在
if(!file_exists($file_path))
{
      echo '对不起,你要下载的文件不存在。';
      return false;
}
$file_size = filesize($file_path);
header("Content-type: application/octet-stream");
header("Accept-Ranges: bytes");
header("Accept-Length: $file_size");
header("Content-Disposition: attachment; filename=".$file_name);
$fp = fopen($file_path,"r");
$buffer_size = 1024;
$cur_pos = 0;
while(!feof($fp)&&$file_size-$cur_pos>$buffer_size)
{
     $buffer = fread($fp,$buffer_size);
     echo $buffer;
     $cur_pos += $buffer_size;
}
$buffer = fread($fp,$file_size-$cur_pos);
echo $buffer;
fclose($fp);
return true;
}