分享一个超好用的php header下载函数

<?php
/**
 * 发送文件
 *
 * @author: legend(legendsky@hotmail.com)
 * @link: http://www.ugia.cn/?p=109
 * @description: send file to client
 * @version: 1.0
 *
 * @param string   $filename      文件名称或路径
 * @param string   $fancyname     自定义的文件名,为空则使用filename
 * @param boolean  $forcedownload 是否强制下载
 * @param integer  $speedlimit    速度限制,单位为字节,0为不限制,不支持windows服务器
 * @param string   $$contenttype  文件类型,默认为application/octet-stream
 *
 * @return boolean
 */
function sendfile($filename, $fancyname = '', $forcedownload = true, $speedlimit = 0, $contenttype = '')
{
    if (!is_readable($filename))
    {
        header("http/1.1 404 not found");
        return false;
    }

    $filestat = stat($filename);
    $lastmodified = $filestat['mtime'];

    $md5 = md5($filestat['mtime'] .'='. $filestat['ino'] .'='. $filestat['size']);
    $etag = '"' . $md5 . '-' . crc32($md5) . '"';

    header('last-modified: ' . gmdate("d, d m y h:i:s", $lastmodified) . ' gmt');
    header("etag: $etag");

    if (isset($_server['http_if_modified_since']) && strtotime($_server['http_if_modified_since']) >= $lastmodified)
    {
        header("http/1.1 304 not modified");
        return true;
    }

    if (isset($_server['http_if_unmodified_since']) && strtotime($_server['http_if_unmodified_since']) < $lastmodified)
    {
        header("http/1.1 304 not modified");
        return true;
    }

    if (isset($_server['http_if_none_match']) &&  $_server['http_if_none_match'] == $etag)
    {
        header("http/1.1 304 not modified");
        return true;
    }

    if ($fancyname == '')
    {
        $fancyname = basename($filename);
    }

    if ($contenttype == '')
    {
        $contenttype = 'application/octet-stream';
    }

    $filesize = $filestat['size'];  

    $contentlength = $filesize;
    $ispartial = false;

    if (isset($_server['http_range']))
    {
        if (preg_match('/^bytes=(d*)-(d*)$/', $_server['http_range'], $matches))
        {   
            $startpos = $matches[1];
            $endpos = $matches[2];

            if ($startpos == '' && $endpos == '')
            {
                return false;
            }

            if ($startpos == '')
            {
                $startpos = $filesize - $endpos;
                $endpos = $filesize - 1;
            }
            else if ($endpos == '')
            {
                $endpos = $filesize - 1;
            }

            $startpos = $startpos < 0 ? 0 : $startpos;
            $endpos = $endpos > $filesize - 1 ? $filesize - 1 : $endpos;

            $length = $endpos - $startpos + 1;

            if ($length < 0)
            {
                return false;
            }

            $contentlength = $length;
            $ispartial = true;
        }
    }

    // send headers
    if ($ispartial)
    {
        header('http/1.1 206 partial content');
        header("content-range: bytes $startpos-$endpos/$filesize");

    }
    else
    {
        header("http/1.1 200 ok");
        $startpos = 0;
        $endpos = $contentlength - 1;
    }

    header('pragma: cache');
    header('cache-control: public, must-revalidate, max-age=0');
    header('accept-ranges: bytes');
    header('content-type: ' . $contenttype);
    header('content-length: ' . $contentlength);

    if ($forcedownload)
    {
        header('content-disposition: attachment; filename="' . rawurlencode($fancyname). '"');//汉字自动转为url编码
  header('content-disposition: attachment; filename="' . $fancyname. '"');
    }

    header("content-transfer-encoding: binary");

    $buffersize = 2048;

    if ($speedlimit != 0)
    {
        $packettime = floor($buffersize * 1000000 / $speedlimit);
    }

    $bytessent = 0;
    $fp = fopen($filename, "rb");
    fseek($fp, $startpos);

    //fpassthru($fp);

    while ($bytessent < $contentlength && !feof($fp) && connection_status() == 0 )
    {
        if ($speedlimit != 0)
        {
            list($usec, $sec) = explode(" ", microtime());
            $outputtimestart = ((float)$usec + (float)$sec);
        }

        $readbuffersize = $contentlength - $bytessent < $buffersize ? $contentlength - $bytessent : $buffersize;
        $buffer = fread($fp, $readbuffersize);       

        echo $buffer;

        ob_flush();
        flush();

        $bytessent += $readbuffersize;

        if ($speedlimit != 0)
        {
            list($usec, $sec) = explode(" ", microtime());
            $outputtimeend = ((float)$usec + (float)$sec);

            $usetime = ((float) $outputtimeend - (float) $outputtimestart) * 1000000;
            $sleeptime = round($packettime - $usetime);
            if ($sleeptime > 0)
            {
                usleep($sleeptime);
            }
        }
    }
   

    return true;
}
 ?>