XML : XMLWriter return a empty string

I have a public function to construct a xml based on an object array:

  $writer = new \XMLWriter();      $writer->openMemory();      $writer->setIndentString("");      $writer->setIndent(false);      $writer->startDocument('1.0', 'UTF-8');      $writer->startElement('correioslog');      $writer->writeElement('tipo_arquivo', 'Postagem');      $writer->writeElement('versao_arquivo', '2.3');      $this->writePlp($writer, $data);      $this->writeRemetente($writer, $data);      $this->writeFormaPagamento($writer, $data);      foreach ($data->getEncomendas() as $objetoPostal) {          $this->writeObjetoPostal($writer, $objetoPostal);      }      $writer->endElement();      $writer->endDocument();      return $writer;    

But the result of this is a empty string:

  object(XMLWriter)#70 (0) {}    

Can anyone help me?