Include custom HTTP header for each request

First add the EventListener into your bundle to react to Response events:

<?php
namespace Acme\DemoBundle\EventListener;

use \Symfony\Component\HttpKernel\Event\FilterResponseEvent;

class ResponseListener
{
    public function onKernelRequest(FilterResponseEvent $event)
    {
        $response = $event->getResponse();
        $response->headers->set('X-Custom', 'test');
        $event->setResponse($response);
    }
}

then register it in DemoBundle/Resources/config/services.yml (or app/config/services.yml for all bundles)

services:
    acme.event_listener.response_listener:
        class: Acme\DemoBundle\EventListener\ResponseListener
        tags:
            - { name: kernel.event_listener, event: kernel.response, method: onKernelRequest }

Can be useful in test environment to set the latest Git revision (ATTENTION: performance killer):

$response->headers->set('X-Git-Rev', exec('git rev-parse --verify HEAD 2> /dev/null'));