Silex - Best Practice for Client Choice Response Format (JSON / XML) - php

Question

I would like to know the best coding practice that allows the client to define the response format for the request he made, which can also include filters, conditions, sorts, etc ...
I made a small template for an answer, I do not know if it is a best practice, but it works (advice is welcome). I coded no Middleware after();
PS: The choice of format will be dynamic after building the request code.
Now about a requisition. I figured I'd code any middleware before(). What is the best way to do it?
Follow the code:
index.php
<?php
define('ROOT', dirname(__DIR__));
chdir(ROOT);
require 'vendor/autoload.php';
require 'src/Config/bootstrap.php';
require 'src/Config/routes.php';
$app->run();
bootstrap.php
<?php
use Silex\Application;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Serializer\Serializer;
use Symfony\Component\Serializer\Encoder\XmlEncoder;
use Symfony\Component\Serializer\Encoder\JsonEncoder;
use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;
$app = new Application();
$app['serializer'] = function(){
$encoders = array(new XmlEncoder(), new JsonEncoder());
$normalizers = array(new ObjectNormalizer());
return new Serializer($normalizers, $encoders);
};
$app['debug'] = true;
/*$app->before(function (Request $request) use ($app){
$request->query->
});*/
$app->after(function (Request $request, Response $response) use ($app){
//var_dump($response);
$response->headers->set('Content-Type', 'application/xml');
return $response;
});
return $app;
routes.php
<?php
$app->mount('/classificados', require 'src/App/Controllers/ClassificadosController.php');
ClassificadosController.php
<?php
use Symfony\Component\HttpFoundation\Response;
$classificados = $app['controllers_factory'];
$classificados->get('/', function() use ($app) {
$post = array(
'title' => 'Titulo',
'body' => 'corpo',
);
$serializeContent = $app['serializer']->serialize($post, 'xml');
return new Response($serializeContent, 200);
});
return $classificados;
What is the best way to build a logic to dynamize the response format (json or xml) for the client?