Box\Spout\Writer\XLSX\Helper\FileSystemHelper::createWorkbookRelsFile PHP Method

    public function createWorkbookRelsFile($worksheets)
    {
        $workbookRelsXmlFileContents = <<<EOD
<?xml version="1.0" encoding="UTF-8"?>
<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
    <Relationship Id="rIdStyles" Target="styles.xml" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles"/>
    <Relationship Id="rIdSharedStrings" Target="sharedStrings.xml" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings"/>
EOD;
        /** @var Worksheet $worksheet */
        foreach ($worksheets as $worksheet) {
            $worksheetId = $worksheet->getId();
            $workbookRelsXmlFileContents .= '<Relationship Id="rIdSheet' . $worksheetId . '" Target="worksheets/sheet' . $worksheetId . '.xml" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet"/>';
        }
        $workbookRelsXmlFileContents .= '</Relationships>';
        $this->createFileWithContents($this->xlRelsFolder, self::WORKBOOK_RELS_XML_FILE_NAME, $workbookRelsXmlFileContents);
        return $this;
    }