Add custom items to menu in Wordpress #php

add_custom_items_to_wp_menu.php

<?php
/* add custom items to primary menu */
add_filter('wp_nav_menu_items','add_search_box_to_menu', 10, 2);
function add_search_box_to_menu( $items, $args ) {
    if( $args->theme_location == 'primary' ) /* ID of menu */
	     return $items.'CUSTOM THINGS';
    return $items;
}