Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xls/Workbook.php
private function writeExternalsheetBiff8()
    {
        $totalReferences = count($this->parser->references);
        $record = 0x0017; // Record identifier
        $length = 2 + 6 * $totalReferences; // Number of bytes to follow

        $supbook_index = 0; // FIXME: only using internal SUPBOOK record
        $header = pack('vv', $record, $length);
        $data = pack('v', $totalReferences);
        for ($i = 0; $i < $totalReferences; ++$i) {
            $data .= $this->parser->references[$i];
        }

        return $this->writeData($header . $data);
    }
private function writeExternalsheetBiff8()
    {
        $totalReferences = count($this->parser->references);
        $record = 0x0017; // Record identifier
        $length = 2 + 6 * $totalReferences; // Number of bytes to follow

        $supbook_index = 0; // FIXME: only using internal SUPBOOK record
        $header = pack('vv', $record, $length);
        $data = pack('v', $totalReferences);
        for ($i = 0; $i < $totalReferences; ++$i) {
            $data .= $this->parser->references[$i];
        }

        return $this->writeData($header . $data);
    }
Writes the Excel BIFF EXTERNSHEET record. These references are used by
formulas.