login-logout-menu-items.php

<?php
/**
 * Add login and logout links to main menu
 *
 * @param string $items The HTML list content for the menu items.
 * @param obj $args An object containing wp_nav_menu() arguments.
 *
 * @link https://codex.wordpress.org/Function_Reference/wp_login_url
 * @link https://codex.wordpress.org/Function_Reference/wp_logout_url
 * @link https://developer.wordpress.org/reference/functions/wp_nav_menu/
 * @link https://developer.wordpress.org/reference/hooks/wp_nav_menu_items/
 */
function frontend_login_logout_menu_items( $items, $args ) {
   if ($args->theme_location == 'primary') {
      if ( is_user_logged_in() ) {
         $items .= '<li class="menu-item"><a href="' . wp_logout_url( home_url() ) . '">Logout</a></li>';
      } else {
         $items .= '<li class="menu-item"><a href="' . wp_login_url( get_permalink() ) . '" title="Login">Login</a></li>';
      }
   }
   return $items;
}
add_filter( 'wp_nav_menu_items', 'frontend_login_logout_menu_items', 10, 2 );