Wordpress Custom Menu

functions.php

<?php

/**
 * Split the menu to top level items and the children of the
 * current active parent.
 * @see http://helperclass.blogspot.de/2011/11/creating-separate-sub-menu-in-wordpress.html
 */
class SplitMenu_Walker_Nav_Menu extends Walker_Nav_Menu {
    private $menu = false;

    public function start_lvl(&$output, $depth) {
        return false;
    }

    public function end_lvl($output, $depth) {
        $this->menu = false;
    }

    public function start_el(&$output, $item, $depth, $args) {
        if(($args->has_children && $item->current) || $item->current_item_parent) {
            $this->menu = true;
        }
        
        if($this->menu && $depth > 0){
            parent::start_el($output, $item, $depth, $args);
        }
    }

    public function end_el(&$output, $item, $depth) {
        if( $this->menu ) {
            parent::end_el($output, $item, $depth);
        }
    }

    public function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {
        $id = $this->db_fields['id'];
        if ( is_object( $args[0] ) ) {
            $args[0]->has_children = ! empty( $children_elements[$element->$id] );
        }
        return parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }
}

header.php

<?php

  /**
    * Get custom menu (named: context)
    * and list only first level items (depth=1)
    */
    $nav_menu_config = array(
        'menu' => 'context',
        'depth'=> 1
    );
    wp_nav_menu($nav_menu_config);

sidebar.php

<?php

   /**
    * Get custom menu (named: context)	and list only
    * items matched by SplitMenu_Walker_Nav_Menu
    */
    $nav_menu_config = array(
        'menu'   => 'context',
        'walker' => new SplitMenu_Walker_Nav_Menu
    );
    wp_nav_menu($nav_menu_config);