Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xls/BIFFwriter.php
protected function append($data)
    {
        if (strlen($data) - 4 > $this->limit) {
            $data = $this->addContinue($data);
        }
        $this->_data .= $data;
        $this->_datasize += strlen($data);
    }
protected function append($data)
    {
        if (strlen($data) - 4 > $this->limit) {
            $data = $this->addContinue($data);
        }
        $this->_data .= $data;
        $this->_datasize += strlen($data);
    }
General storage function.

@param string $data binary data to append