PHPOffice / PhpSpreadsheet

     * @param string  $format Custom format string

     * @param integer $ifmt   Format index code

     */

    private function writeNumberFormat($format, $ifmt)

    {

        $record = 0x041E;    // Record identifier

        $numberFormatString = \PhpSpreadsheet\Shared\StringHelper::UTF8toBIFF8UnicodeLong($format);

        $length = 2 + strlen($numberFormatString);    // Number of bytes to follow

        $header = pack("vv", $record, $length);

        $data   = pack("v", $ifmt) .  $numberFormatString;

        $this->append($header . $data);

    }

    /**

     * Write DATEMODE record to indicate the date system in use (1904 or 1900).

     *

     * @param string $sheetname Worksheet name

     */

    private function writeExternalSheet($sheetname)

    {

        $record = 0x0017;                     // Record identifier

        $length = 0x02 + strlen($sheetname);  // Number of bytes to follow

        $cch    = strlen($sheetname);         // Length of sheet name

        $rgch   = 0x03;                       // Filename encoding

        $header = pack("vv", $record, $length);

        $data   = pack("CC", $cch, $rgch);

        $this->append($header . $data . $sheetname);

    }

    /**

     * Store the NAME record in the short format that is used for storing the print