[wordpress] Add login/logout link to wordpress menu based on user login status

functions.php

<?php
// ADD LOGIN / LOGOUT FUNCTION TO MENU
add_filter('wp_nav_menu_items', 'add_login_logout_link', 10, 2);
function add_login_logout_link($items, $args) {
  ob_start();
  wp_loginout('index.php');
  $loginoutlink = ob_get_contents();
  ob_end_clean();
  $items .= '<li>'. $loginoutlink .'</li>';     return $items;
}