spout type cell string||bool||number Worksheet:getCellXML

 /**
     * Build and return xml for a single cell.
     *
     * @param int $rowIndex
     * @param int $cellNumber
     * @param mixed $cellValue
     * @param int $styleId
     * @return string
     * @throws InvalidArgumentException If the given value cannot be processed
     */
    protected function getCellXML($rowIndex, $cellNumber, $cellValue, $styleId)
    {
        $columnIndex = CellHelper::getCellIndexFromColumnIndex($cellNumber);
        $cellXML = '<c r="' . $columnIndex . $rowIndex . '"';
        $cellXML .= ' s="' . $styleId . '"';

        if (CellHelper::isNonEmptyString($cellValue)) {/* return (gettype($value) === 'string' && $value !== '');*/
            $cellXML .= $this->getCellXMLFragmentForNonEmptyString($cellValue);
        } else if (CellHelper::isBoolean($cellValue)) {
            $cellXML .= ' t="b"><v>' . intval($cellValue) . '</v></c>';
        } else if (CellHelper::isNumeric($cellValue)) {
            $cellXML .= '><v>' . $cellValue . '</v></c>';
        } else if (empty($cellValue)) {
            if ($this->styleHelper->shouldApplyStyleOnEmptyCell($styleId)) {
                $cellXML .= '/>';
            } else {
                // don't write empty cells that do no need styling
                // NOTE: not appending to $cellXML is the right behavior!!
                $cellXML = '';
            }
        } else {
            throw new InvalidArgumentException('Trying to add a value with an unsupported type: ' . gettype($cellValue));
        }

        return $cellXML;
    }