Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Reader/Xls.php
private function readCodepage()
    {
        $length = self::getUInt2d($this->data, $this->pos + 2);
        $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);

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

        // offset: 0; size: 2; code page identifier
        $codepage = self::getUInt2d($recordData, 0);

        $this->codepage = CodePage::numberToName($codepage);
    }
private function readCodepage()
    {
        $length = self::getUInt2d($this->data, $this->pos + 2);
        $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);

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

        // offset: 0; size: 2; code page identifier
        $codepage = self::getUInt2d($recordData, 0);

        $this->codepage = CodePage::numberToName($codepage);
    }
CODEPAGE.

This record stores the text encoding used to write byte
strings, stored as MS Windows code page identifier.

--    "OpenOffice.org's Documentation of the Microsoft
Excel File Format"