Вот это
$response = $responseFactory->createResponse();
$response = $response->getBody()->write('test1');
$dataResponse = new DataResponse(['test' => 'test2'], $response);
$dataResponse->withDataForamatter(JsonDataFormatter);
$dataResponse->getBody()->rewind();
$dataResponse->getBody()->getContents();
Выдаст строку
test1{"test":"test2"}