eZ\Publish\Core\REST\Common\Output\Generator\Xml::startDocument PHP Method

    public function startDocument($data)
    {
        $this->checkStartDocument($data);
        $this->isEmpty = true;
        $this->xmlWriter = new \XMLWriter();
        $this->xmlWriter->openMemory();
        $this->xmlWriter->setIndent($this->formatOutput);
        $this->xmlWriter->startDocument('1.0', 'UTF-8');
    }