Render XML-Data using Twig-Template within the SILEX-Microframework

index.php

<?php 
// Add to use-statements
use Symfony\Component\HttpFoundation\Response;

// [...] Your application code

// Action to return XML from Twig-Template
$app->get(
    '/sitemap.xml',
    function () use ($app) {
        $data = array();

        return new Response($app['twig']->render(
                'sitemap.xml.twig', // Your twig-template eg. sitemap.xml.twig
                array('data' => $data)
            ),
            200,
            array('Content-Type' => 'application/xml')
        );
    }
);