Useful php header download function

/**
 * Send file
 *
 * @param string $fileName File name or path
 * @param string $fancyName Customized file name, if empty, use filename
 * @param boolean $forceDownload Whether to force download
 * @param integer $speedLimit speed limit, unit is bytes, 0 means no limit, does not support windows server
 * @param string $$contentType file type, the default is 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");
    // header函数实现文件下载
    $bufferSize = 2048;
    if ($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);
        } // bbs.it-home.org
        $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;
}
?>