layout.html.twig

{{ knp_menu_render('main') }}

MenuBuilder.php

<?php

namespace AppBundle\Menu;

use Knp\Menu\FactoryInterface;
use Symfony\Component\HttpFoundation\RequestStack;

class MenuBuilder
{
    private $factory;

    /**
     * @param FactoryInterface $factory
     */
    public function __construct(FactoryInterface $factory)
    {
        $this->factory = $factory;
    }


    public function createMainMenu(RequestStack $requestStack)
    {
        $menu = $this->factory->createItem('root');

        $menu->addChild('level1', ['uri' => '#']);

        $menu['level1']->addChild('level2', ['uri' => '#' ]);

        $menu['level2']->addChild('level3', ['uri' => '#' ]);

        return $menu;
    }

}

services.yml

knp_menu:
    # use "twig: false" to disable the Twig extension and the TwigRenderer
    twig:
        template: knp_menu.html.twig
    #  if true, enables the helper for PHP templates
    templating: false
    # the renderer to use, list is also available by default
    default_renderer: twig
    
services:
    app.menu_builder:
        class: AppBundle\Menu\MenuBuilder
        arguments: ["@knp_menu.factory"]

    app.main_menu:
        class: Knp\Menu\MenuItem # the service definition requires setting the class
        factory: ["@app.menu_builder", createMainMenu]
        arguments: ["@request_stack"]
        tags:
            - { name: knp_menu.menu, alias: main } # The alias is what is used to retrieve the menu