menu.php

<?php

function menu_get_Links_source($name, $default) {
  $config = \Drupal::config('menu.settings');
  // When menu module is not enabled, we need a hardcoded default value.
  return module_exists('menu') ? $config->get('main_links') : 'main';
}

/**
 * Returns an array of links to be rendered as the Main menu.
 */
function menu_main_menu() {
  $main_links_source = menu_get_Links_source('main_links', 'main');
  return menu_navigation_links($main_links_source);
}

/**
 * Returns an array of links to be rendered as the Secondary links.
 */
function menu_secondary_menu() {
  $main_links_source = menu_get_Links_source('main_links', 'main');
  $secondary_links_source = menu_get_Links_source('secondary_links', 'account');

  // If the secondary menu source is set as the primary menu, we display the
  // second level of the primary menu.
  if ($secondary_links_source == $main_links_source) {
    return menu_navigation_links($main_links_source, 1);
  }
  else {
    return menu_navigation_links($secondary_links_source, 0);
  }
}