Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xlsx/Rels.php
public function writeRelationships(Spreadsheet $spreadsheet)
    {
        // Create XML writer
        $objWriter = null;
        if ($this->getParentWriter()->getUseDiskCaching()) {
            $objWriter = new XMLWriter(XMLWriter::STORAGE_DISK, $this->getParentWriter()->getDiskCachingDirectory());
        } else {
            $objWriter = new XMLWriter(XMLWriter::STORAGE_MEMORY);
        }

        // XML header
        $objWriter->startDocument('1.0', 'UTF-8', 'yes');

        // Relationships
        $objWriter->startElement('Relationships');
        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');

        $customPropertyList = $spreadsheet->getProperties()->getCustomProperties();
        if (!empty($customPropertyList)) {
            // Relationship docProps/app.xml
            $this->writeRelationship(
                $objWriter,
                4,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/custom-properties',
                'docProps/custom.xml'
            );
        }

        // Relationship docProps/app.xml
        $this->writeRelationship(
            $objWriter,
            3,
            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/extended-properties',
            'docProps/app.xml'
        );

        // Relationship docProps/core.xml
        $this->writeRelationship(
            $objWriter,
            2,
            'http://schemas.openxmlformats.org/package/2006/relationships/metadata/core-properties',
            'docProps/core.xml'
        );

        // Relationship xl/workbook.xml
        $this->writeRelationship(
            $objWriter,
            1,
            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument',
            'xl/workbook.xml'
        );
        // a custom UI in workbook ?
        if ($spreadsheet->hasRibbon()) {
            $this->writeRelationShip(
                $objWriter,
                5,
                'http://schemas.microsoft.com/office/2006/relationships/ui/extensibility',
                $spreadsheet->getRibbonXMLData('target')
            );
        }

        $objWriter->endElement();

        return $objWriter->getData();
    }
public function writeRelationships(Spreadsheet $spreadsheet)
    {
        // Create XML writer
        $objWriter = null;
        if ($this->getParentWriter()->getUseDiskCaching()) {
            $objWriter = new XMLWriter(XMLWriter::STORAGE_DISK, $this->getParentWriter()->getDiskCachingDirectory());
        } else {
            $objWriter = new XMLWriter(XMLWriter::STORAGE_MEMORY);
        }

        // XML header
        $objWriter->startDocument('1.0', 'UTF-8', 'yes');

        // Relationships
        $objWriter->startElement('Relationships');
        $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');

        $customPropertyList = $spreadsheet->getProperties()->getCustomProperties();
        if (!empty($customPropertyList)) {
            // Relationship docProps/app.xml
            $this->writeRelationship(
                $objWriter,
                4,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/custom-properties',
                'docProps/custom.xml'
            );
        }

        // Relationship docProps/app.xml
        $this->writeRelationship(
            $objWriter,
            3,
            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/extended-properties',
            'docProps/app.xml'
        );

        // Relationship docProps/core.xml
        $this->writeRelationship(
            $objWriter,
            2,
            'http://schemas.openxmlformats.org/package/2006/relationships/metadata/core-properties',
            'docProps/core.xml'
        );

        // Relationship xl/workbook.xml
        $this->writeRelationship(
            $objWriter,
            1,
            'http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument',
            'xl/workbook.xml'
        );
        // a custom UI in workbook ?
        if ($spreadsheet->hasRibbon()) {
            $this->writeRelationShip(
                $objWriter,
                5,
                'http://schemas.microsoft.com/office/2006/relationships/ui/extensibility',
                $spreadsheet->getRibbonXMLData('target')
            );
        }

        $objWriter->endElement();

        return $objWriter->getData();
    }
Write relationships to XML format.

@param Spreadsheet $spreadsheet

@throws WriterException

@return string XML Output