Defining an event listener that adds a HTML comment to the Response body - http://www.symfonycentral.com

front.php

<?php
require_once __DIR__.'/../vendor/.composer/autoload.php';

// ... etc
use Symfony\Component\EventDispatcher\EventDispatcher;

$dispatcher = new EventDispatcher();
$dispatcher->addListener('response', function (Simplex\ResponseEvent $event) {
    $response = $event->getResponse();

    $response->setContent($response->getContent().'<!-- ' . time() . ' -->');
});