php怎么实现下载断点续传

/**
 * 文件下载
 * @param string $file 文件绝对路径
 */
function download($file)
{
    str_replace(['/','\'], DIRECTORY_SEPARATOR, $file);
    //检查文件是否存在
    if (empty($file) or !is_file($file)) {
        die('文件不存在');
    }
    $fileName = basename($file);
    //以只读和二进制模式打开文件
    $fp = @fopen($file, 'rb');
    if ($fp) {
        // 获取文件大小
        $file_size = filesize($file);
        //告诉浏览器这是一个文件流格式的文件
        header('content-type:application/octet-stream');
        header('Content-Disposition: attachment; filename=' . $fileName);
        // 断点续传
        $range = null;
        if (!empty($_SERVER['HTTP_RANGE'])) {
            $range = $_SERVER['HTTP_RANGE'];
            $range = preg_replace('/[s|,].*/', '', $range);
            $range = explode('-', substr($range, 6));
            if (count($range) < 2) {
                $range[1] = $file_size;
            }
            $range = array_combine(array('start', 'end'), $range);
            if (empty($range['start'])) {
                $range['start'] = 0;
            }
            if (empty($range['end'])) {
                $range['end'] = $file_size;
            }
        }
        // 使用续传
        if ($range != null) {
            header('HTTP/1.1 206 Partial Content');
            header('Accept-Ranges:bytes');
            // 计算剩余长度
            header(sprintf('content-length:%u', $range['end'] - $range['start']));
            header(sprintf('content-range:bytes %s-%s/%s', $range['start'], $range['end'], $file_size));
            // fp指针跳到断点位置
            fseek($fp, sprintf('%u', $range['start']));
        } else {
            header('HTTP/1.1 200 OK');
            header('Accept-Ranges:bytes');
            header('content-length:' . $file_size);
        }
        while (!feof($fp)) {
            echo fread($fp, 4096);
            ob_flush();
        }
        fclose($fp);
    } else {
        die('File loading failed');
    }
}