Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Reader/Xls.php
private static function readBIFF8ConstantArray($arrayData)
    {
        // offset: 0; size: 1; number of columns decreased by 1
        $nc = ord($arrayData[0]);

        // offset: 1; size: 2; number of rows decreased by 1
        $nr = self::getUInt2d($arrayData, 1);
        $size = 3; // initialize
        $arrayData = substr($arrayData, 3);

        // offset: 3; size: var; list of ($nc + 1) * ($nr + 1) constant values
        $matrixChunks = [];
        for ($r = 1; $r <= $nr + 1; ++$r) {
            $items = [];
            for ($c = 1; $c <= $nc + 1; ++$c) {
                $constant = self::readBIFF8Constant($arrayData);
                $items[] = $constant['value'];
                $arrayData = substr($arrayData, $constant['size']);
                $size += $constant['size'];
            }
            $matrixChunks[] = implode(',', $items); // looks like e.g. '1,"hello"'
        }
        $matrix = '{' . implode(';', $matrixChunks) . '}';

        return [
            'value' => $matrix,
            'size' => $size,
        ];
    }
private static function readBIFF8ConstantArray($arrayData)
    {
        // offset: 0; size: 1; number of columns decreased by 1
        $nc = ord($arrayData[0]);

        // offset: 1; size: 2; number of rows decreased by 1
        $nr = self::getUInt2d($arrayData, 1);
        $size = 3; // initialize
        $arrayData = substr($arrayData, 3);

        // offset: 3; size: var; list of ($nc + 1) * ($nr + 1) constant values
        $matrixChunks = [];
        for ($r = 1; $r <= $nr + 1; ++$r) {
            $items = [];
            for ($c = 1; $c <= $nc + 1; ++$c) {
                $constant = self::readBIFF8Constant($arrayData);
                $items[] = $constant['value'];
                $arrayData = substr($arrayData, $constant['size']);
                $size += $constant['size'];
            }
            $matrixChunks[] = implode(',', $items); // looks like e.g. '1,"hello"'
        }
        $matrix = '{' . implode(';', $matrixChunks) . '}';

        return [
            'value' => $matrix,
            'size' => $size,
        ];
    }
read BIFF8 constant value array from array data
returns e.g. ['value' => '{1,2;3,4}', 'size' => 40]
section 2.5.8.

@param string $arrayData

@return array