Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xlsx/Rels.php
public function writeWorksheetRelationships(\PhpOffice\PhpSpreadsheet\Worksheet\Worksheet $pWorksheet, $pWorksheetId = 1, $includeCharts = false)
    {
        // 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');

        // Write drawing relationships?
        $d = 0;
        if ($includeCharts) {
            $charts = $pWorksheet->getChartCollection();
        } else {
            $charts = [];
        }
        if (($pWorksheet->getDrawingCollection()->count() > 0) ||
            (count($charts) > 0)) {
            $this->writeRelationship(
                $objWriter,
                ++$d,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/drawing',
                '../drawings/drawing' . $pWorksheetId . '.xml'
            );
        }

        // Write hyperlink relationships?
        $i = 1;
        foreach ($pWorksheet->getHyperlinkCollection() as $hyperlink) {
            if (!$hyperlink->isInternal()) {
                $this->writeRelationship(
                    $objWriter,
                    '_hyperlink_' . $i,
                    'http://schemas.openxmlformats.org/officeDocument/2006/relationships/hyperlink',
                    $hyperlink->getUrl(),
                    'External'
                );

                ++$i;
            }
        }

        // Write comments relationship?
        $i = 1;
        if (count($pWorksheet->getComments()) > 0) {
            $this->writeRelationship(
                $objWriter,
                '_comments_vml' . $i,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing',
                '../drawings/vmlDrawing' . $pWorksheetId . '.vml'
            );

            $this->writeRelationship(
                $objWriter,
                '_comments' . $i,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/comments',
                '../comments' . $pWorksheetId . '.xml'
            );
        }

        // Write header/footer relationship?
        $i = 1;
        if (count($pWorksheet->getHeaderFooter()->getImages()) > 0) {
            $this->writeRelationship(
                $objWriter,
                '_headerfooter_vml' . $i,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing',
                '../drawings/vmlDrawingHF' . $pWorksheetId . '.vml'
            );
        }

        $objWriter->endElement();

        return $objWriter->getData();
    }
public function writeWorksheetRelationships(\PhpOffice\PhpSpreadsheet\Worksheet\Worksheet $pWorksheet, $pWorksheetId = 1, $includeCharts = false)
    {
        // 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');

        // Write drawing relationships?
        $d = 0;
        if ($includeCharts) {
            $charts = $pWorksheet->getChartCollection();
        } else {
            $charts = [];
        }
        if (($pWorksheet->getDrawingCollection()->count() > 0) ||
            (count($charts) > 0)) {
            $this->writeRelationship(
                $objWriter,
                ++$d,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/drawing',
                '../drawings/drawing' . $pWorksheetId . '.xml'
            );
        }

        // Write hyperlink relationships?
        $i = 1;
        foreach ($pWorksheet->getHyperlinkCollection() as $hyperlink) {
            if (!$hyperlink->isInternal()) {
                $this->writeRelationship(
                    $objWriter,
                    '_hyperlink_' . $i,
                    'http://schemas.openxmlformats.org/officeDocument/2006/relationships/hyperlink',
                    $hyperlink->getUrl(),
                    'External'
                );

                ++$i;
            }
        }

        // Write comments relationship?
        $i = 1;
        if (count($pWorksheet->getComments()) > 0) {
            $this->writeRelationship(
                $objWriter,
                '_comments_vml' . $i,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing',
                '../drawings/vmlDrawing' . $pWorksheetId . '.vml'
            );

            $this->writeRelationship(
                $objWriter,
                '_comments' . $i,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/comments',
                '../comments' . $pWorksheetId . '.xml'
            );
        }

        // Write header/footer relationship?
        $i = 1;
        if (count($pWorksheet->getHeaderFooter()->getImages()) > 0) {
            $this->writeRelationship(
                $objWriter,
                '_headerfooter_vml' . $i,
                'http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing',
                '../drawings/vmlDrawingHF' . $pWorksheetId . '.vml'
            );
        }

        $objWriter->endElement();

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

Numbering is as follows:
rId1                 - Drawings
rId_hyperlink_x     - Hyperlinks

@param \PhpOffice\PhpSpreadsheet\Worksheet\Worksheet $pWorksheet
@param int $pWorksheetId
@param bool $includeCharts Flag indicating if we should write charts

@throws WriterException

@return string XML Output