Bytes转到各种MB,GB等等的方法

//单位转换，G和MB转换
        if ($length >= 1073741824) {
            $length = round($length / 1073741824 * 100) / 100 . ' GB';
        } elseif ($length >= 1048576) {
            $length = round($length / 1048576 * 100) / 100 . ' MB';
        } elseif ($length >= 1024) {
            $length = round($length / 1024 * 100) / 100 . ' KB';
        } else {
            $length = $length . ' Bytes';
        }