if( ! function_exists( 'videotube_add_searchform_on_menu' ) ){
    /**
     * @param unknown_type $items
     * @param unknown_type $args
     * @return string
     */
    function videotube_add_searchform_on_menu( $items, $args ) {
        if ( $args->theme_location == 'header_main_navigation' ) {
            $items .= '<li id="search-form" class="search-form menu-item">'. get_search_form(false) .'</li>';
        }
        return $items;
    }
    add_filter( 'wp_nav_menu_items', 'videotube_add_searchform_on_menu', 10, 2 );
}