functions.php - add search to main menu

<?php

// Exit if accessed directly
if ( !defined('ABSPATH')) exit;

/**
 *
 * Add search to main menu
 *
 */
    add_filter('wp_nav_menu_items','add_search_box', 10, 2);
    function add_search_box($items, $args) {
        if($args->theme_location == 'header-menu') {
            ob_start();
            get_search_form();
            $searchform = ob_get_contents();
            ob_end_clean();

            return $items .= '<li id="searchform-item">' . $searchform . '</li>';
        }
        return $items;
    }