Drupal 7's similarities

Once drupal_bootstrap() is simplified and overridden, the remaining problem to solve is actually handling the request and returning a response.

You might assume that Drupal 7 would require extensive rewrites to even begin to operate within the constraints of the Symfony HttpKernel. Nothing in Drupal 7 uses a Symfony request object, and no page callback returns a Symfony response object.

In Drupal, menu_execute_active_handler() reads the current URL path, searches for a route that matches that path, calls the page callback, and calls another function which renders the result.

<?php
function menu_execute_active_handler($path = NULL, $deliver = TRUE) {
  // Check if site is offline.
  $page_callback_result = _menu_site_is_offline() ? MENU_SITE_OFFLINE : MENU_SITE_ONLINE;

  // Allow other modules to change the site status but not the path because that
  // would not change the global variable. hook_url_inbound_alter() can be used
  // to change the path. Code later will not use the $read_only_path variable.
  $read_only_path = !empty($path) ? $path : $_GET['q'];
  drupal_alter('menu_site_status', $page_callback_result, $read_only_path);

  // Only continue if the site status is not set.
  if ($page_callback_result == MENU_SITE_ONLINE) {
    if ($router_item = menu_get_item($path)) {
      if ($router_item['access']) {
        if ($router_item['include_file']) {
          require_once DRUPAL_ROOT . '/' . $router_item['include_file'];
        }
        $page_callback_result = call_user_func_array($router_item['page_callback'], $router_item['page_arguments']);
      }
      else {
        $page_callback_result = MENU_ACCESS_DENIED;
      }
    }
    else {
      $page_callback_result = MENU_NOT_FOUND;
    }
  }

  // Deliver the result of the page callback to the browser, or if requested,
  // return it raw, so calling code can do more processing.
  if ($deliver) {
    $default_delivery_callback = (isset($router_item) && $router_item) ? $router_item['delivery_callback'] : NULL;
    drupal_deliver_page($page_callback_result, $default_delivery_callback);
  }
  else {
    return $page_callback_result;
  }
}

I'll break this function into three parts:

The beginning with the comment Check if site is offline is something I abandoned entirely. If the site is offline, Symfony or the web server will handle it. (Stack Backstage is an HttpKernel-based alternative that provides similar functionality.)
The middle which begins with the comment Only continue if the site status is not set calls the Drupal page callback.
The end which begins with the comment Deliver the result of the page callback to the browser sends the page callback result to another function called a "delivery callback" which will actually render the page or something to the user.

A near-perfect parallel

menu_execute_active_handler() roughly maps to the Symfony HttpKernelInterface::handle() method. In fact, they both have a significant element in common.

In Drupal's menu_execute_active_handler():

<?php
$page_callback_result = call_user_func_array($router_item['page_callback'], $router_item['page_arguments']);

looks remarkably like HttpKernel::handle():

<?php
$response = call_user_func_array($controller, $arguments);

Because the HttpKernel triggers events at many different points in this function, I can coax Drupal's page callbacks to work like Symfony controllers.