Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xls/Workbook.php
public function addXfWriter(Style $style, $isStyleXf = false)
    {
        $xfWriter = new Xf($style);
        $xfWriter->setIsStyleXf($isStyleXf);

        // Add the font if not already added
        $fontIndex = $this->addFont($style->getFont());

        // Assign the font index to the xf record
        $xfWriter->setFontIndex($fontIndex);

        // Background colors, best to treat these after the font so black will come after white in custom palette
        $xfWriter->setFgColor($this->addColor($style->getFill()->getStartColor()->getRGB()));
        $xfWriter->setBgColor($this->addColor($style->getFill()->getEndColor()->getRGB()));
        $xfWriter->setBottomColor($this->addColor($style->getBorders()->getBottom()->getColor()->getRGB()));
        $xfWriter->setTopColor($this->addColor($style->getBorders()->getTop()->getColor()->getRGB()));
        $xfWriter->setRightColor($this->addColor($style->getBorders()->getRight()->getColor()->getRGB()));
        $xfWriter->setLeftColor($this->addColor($style->getBorders()->getLeft()->getColor()->getRGB()));
        $xfWriter->setDiagColor($this->addColor($style->getBorders()->getDiagonal()->getColor()->getRGB()));

        // Add the number format if it is not a built-in one and not already added
        if ($style->getNumberFormat()->getBuiltInFormatCode() === false) {
            $numberFormatHashCode = $style->getNumberFormat()->getHashCode();

            if (isset($this->addedNumberFormats[$numberFormatHashCode])) {
                $numberFormatIndex = $this->addedNumberFormats[$numberFormatHashCode];
            } else {
                $numberFormatIndex = 164 + count($this->numberFormats);
                $this->numberFormats[$numberFormatIndex] = $style->getNumberFormat();
                $this->addedNumberFormats[$numberFormatHashCode] = $numberFormatIndex;
            }
        } else {
            $numberFormatIndex = (int) $style->getNumberFormat()->getBuiltInFormatCode();
        }

        // Assign the number format index to xf record
        $xfWriter->setNumberFormatIndex($numberFormatIndex);

        $this->xfWriters[] = $xfWriter;

        $xfIndex = count($this->xfWriters) - 1;

        return $xfIndex;
    }
public function addXfWriter(Style $style, $isStyleXf = false)
    {
        $xfWriter = new Xf($style);
        $xfWriter->setIsStyleXf($isStyleXf);

        // Add the font if not already added
        $fontIndex = $this->addFont($style->getFont());

        // Assign the font index to the xf record
        $xfWriter->setFontIndex($fontIndex);

        // Background colors, best to treat these after the font so black will come after white in custom palette
        $xfWriter->setFgColor($this->addColor($style->getFill()->getStartColor()->getRGB()));
        $xfWriter->setBgColor($this->addColor($style->getFill()->getEndColor()->getRGB()));
        $xfWriter->setBottomColor($this->addColor($style->getBorders()->getBottom()->getColor()->getRGB()));
        $xfWriter->setTopColor($this->addColor($style->getBorders()->getTop()->getColor()->getRGB()));
        $xfWriter->setRightColor($this->addColor($style->getBorders()->getRight()->getColor()->getRGB()));
        $xfWriter->setLeftColor($this->addColor($style->getBorders()->getLeft()->getColor()->getRGB()));
        $xfWriter->setDiagColor($this->addColor($style->getBorders()->getDiagonal()->getColor()->getRGB()));

        // Add the number format if it is not a built-in one and not already added
        if ($style->getNumberFormat()->getBuiltInFormatCode() === false) {
            $numberFormatHashCode = $style->getNumberFormat()->getHashCode();

            if (isset($this->addedNumberFormats[$numberFormatHashCode])) {
                $numberFormatIndex = $this->addedNumberFormats[$numberFormatHashCode];
            } else {
                $numberFormatIndex = 164 + count($this->numberFormats);
                $this->numberFormats[$numberFormatIndex] = $style->getNumberFormat();
                $this->addedNumberFormats[$numberFormatHashCode] = $numberFormatIndex;
            }
        } else {
            $numberFormatIndex = (int) $style->getNumberFormat()->getBuiltInFormatCode();
        }

        // Assign the number format index to xf record
        $xfWriter->setNumberFormatIndex($numberFormatIndex);

        $this->xfWriters[] = $xfWriter;

        $xfIndex = count($this->xfWriters) - 1;

        return $xfIndex;
    }
Add a new XF writer.

@param Style $style
@param bool $isStyleXf Is it a style XF?

@return int Index to XF record