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

    protected function createRelsFile()
    {
        $relsFileContents = <<<EOD
<?xml version="1.0" encoding="UTF-8"?>
<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
    <Relationship Id="rIdWorkbook" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument" Target="xl/workbook.xml"/>
    <Relationship Id="rIdCore" Type="http://schemas.openxmlformats.org/officedocument/2006/relationships/metadata/core-properties" Target="docProps/core.xml"/>
    <Relationship Id="rIdApp" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/extended-properties" Target="docProps/app.xml"/>
</Relationships>
EOD;
        $this->createFileWithContents($this->relsFolder, self::RELS_FILE_NAME, $relsFileContents);
        return $this;
    }