Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/TextData.php
public static function CHARACTER($character)
    {
        $character = Functions::flattenSingleValue($character);

        if ((!is_numeric($character)) || ($character < 0)) {
            return Functions::VALUE();
        }

        if (function_exists('iconv')) {
            return iconv('UCS-4LE', 'UTF-8', pack('V', $character));
        }

        return mb_convert_encoding('&#' . (int) $character . ';', 'UTF-8', 'HTML-ENTITIES');
    }
public static function CHARACTER($character)
    {
        $character = Functions::flattenSingleValue($character);

        if ((!is_numeric($character)) || ($character < 0)) {
            return Functions::VALUE();
        }

        if (function_exists('iconv')) {
            return iconv('UCS-4LE', 'UTF-8', pack('V', $character));
        }

        return mb_convert_encoding('&#' . (int) $character . ';', 'UTF-8', 'HTML-ENTITIES');
    }
CHARACTER.

@param string $character Value

@return string