range的官方文档：  http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

range用途：一般是用在断点续传的时候，但是实际的用途很大，例如你的web server需要输出一个大文件，那么用range可以分段输出，缓解压力。同时在提供音乐视频等服务时可以缓冲下载，如果用户中途关闭，可以节约网络带宽。

    <?php
      $filename=$_GET['filename'];
      $location='media/'.$filename;
      
      $extension = substr(strrchr($filename,'.'),1);
      if ($extension == "mp3") {
        $mimeType = "audio/mpeg";
      } else if ($extension == "ogg") {
        $mimeType = "audio/ogg";
      }
      if (!file_exists($location))
      {
        header ("HTTP/1.1 404 Not Found");
        return;
      }
      
      $size  = filesize($location);
      $time  = date('r', filemtime($location));
      
      $fm = @fopen($location, 'rb');
      if (!$fm)
      {
        header ("HTTP/1.1 505 Internal server error");
        return;
      }
      
      $begin  = 0;
      $end  = $size - 1;
      
      if (isset($_SERVER['HTTP_RANGE']))
      {
        if (preg_match('/bytes=\h*(\d+)-(\d*)[\D.*]?/i', $_SERVER['HTTP_RANGE'], $matches))
        {
        $begin  = intval($matches[1]);
        if (!emptyempty($matches[2]))
        {
          $end  = intval($matches[2]);
        }
        }
      }
      if (isset($_SERVER['HTTP_RANGE']))
      {
        header('HTTP/1.1 206 Partial Content');
      }
      else
      {
        header('HTTP/1.1 200 OK');
      }
      
      header("Content-Type: $mimeType");
      header('Cache-Control: public, must-revalidate, max-age=0');
      header('Pragma: no-cache');  
      header('Accept-Ranges: bytes');
      header('Content-Length:' . (($end - $begin) + 1));
      if (isset($_SERVER['HTTP_RANGE']))
      {
        
        header("Content-Range: bytes $begin-$end/$size");
      }
      header("Content-Disposition: inline; filename=$filename");
      header("Content-Transfer-Encoding: binary");
      header("Last-Modified: $time");
      
      $cur  = $begin;
      fseek($fm, $begin, 0);
      
      while(!feof($fm) && $cur <= $end && (connection_status() == 0))
      {
        print fread($fm, min(1024 * 16, ($end - $cur) + 1));
        $cur += 1024 * 16;
      }