private function readBof()
    {
        $length = self::getUInt2d($this->data, $this->pos + 2);
        $recordData = substr($this->data, $this->pos + 4, $length);

        // move stream pointer to next record
        $this->pos += 4 + $length;

        // offset: 2; size: 2; type of the following data
        $substreamType = self::getUInt2d($recordData, 2);

        switch ($substreamType) {
            case self::XLS_WORKBOOKGLOBALS:
                $version = self::getUInt2d($recordData, 0);
                if (($version != self::XLS_BIFF8) && ($version != self::XLS_BIFF7)) {
                    throw new Exception('Cannot read this Excel file. Version is too old.');
                }
                $this->version = $version;

                break;
            case self::XLS_WORKSHEET:
                // do not use this version information for anything
                // it is unreliable (OpenOffice doc, 5.8), use only version information from the global stream
                break;
            default:
                // substream, e.g. chart
                // just skip the entire substream
                do {
                    $code = self::getUInt2d($this->data, $this->pos);
                    $this->readDefault();
                } while ($code != self::XLS_TYPE_EOF && $this->pos < $this->dataSize);

                break;
        }
    }