code: public function indexAction(): InitializeControllerResponse
      {
          @trigger_error('Custom entry points are deprecated and will no longer work in Contao 5.0.', E_USER_DEPRECATED);
  
          $masterRequest = $this->get('request_stack')->getMasterRequest();
  
          if (null === $masterRequest) {
              throw new \RuntimeException('The request stack did not contain a master request.');
          }
  
          $realRequest = Request::createFromGlobals();
  
          // Necessary to generate the correct base path
          foreach (['REQUEST_URI', 'SCRIPT_NAME', 'SCRIPT_FILENAME', 'PHP_SELF'] as $name) {
              $realRequest->server->set(
                  $name,
                  str_replace(TL_SCRIPT, 'index.php', $realRequest->server->get($name))
              );
          }
  
          $realRequest->attributes->replace($masterRequest->attributes->all());
  
          // Initialize the framework with the real request
          $this->get('request_stack')->push($realRequest);
          $this->get('contao.framework')->initialize();
  
          // Add the master request again. When Kernel::handle() is finished,
          // it will pop the current request, resulting in the real request being active.
          $this->get('request_stack')->push($masterRequest);
  
          return new InitializeControllerResponse('', 204);
      }
docs: Initializes the Contao framework.
  
  @throws \RuntimeException
  
  @Route("/_contao/initialize", name="contao_initialize")