Wordpress custom menus are a very valuable feature, as they allow creating hierarchichal menus in the backend. Sometimes it would be useful to just use portions of this menu, e.g when displaying a submenu on a page, containing links to the children of the currently selected parent page in the main menu. Unfortunately Wordpress does not yet provide a simple option for displaying just the children of a particular menu item, so a little more effort is needed to accomplish this task.
Figure out the parent page’s ID
$post_root = 0;
$p = $post;
while( $p->post_parent != 0){
$p = get_post( $p->post_parent );
$post_root = $p;
}
if($post_root == 0)
$post_root = $post;
Get a list of the post root’s children
$children = get_pages( array('child_of' => $post_root->ID ) );
Define a custom Walker which will be used to iterate the menu
class ChildMenuWalker extends Walker_Nav_Menu{
protected $child_ids = array();
protected $exclude = -1;
function __construct($children, $exclude){
$this->child_ids = $children;
$this->exclude = $exclude;
}
function start_el(&$output, $item, $depth, $args){
if( ! in_array($item->object_id, $this->child_ids) ){
return;
} elseif( $item->object_id != $this->exclude) {
parent::start_el(&$output, $item, $depth, $args);
}
}
}
the “start_el” function checks wheter the current item is in the children list. If so, the default Walker’s functionality is used, otherwise nothing is done (output remains empty). Pay attention to use $item->object_id, as this is the post object’s id.
Display the menu with wp_nav_menu using the custom walker class.
wp_nav_menu( array( 'menu' => 'main-menu', 'walker' => new ChildMenuWalker($children_ids, $post_root->ID ) ) )
This renders a menu, containing only the child elements of the selected post root.
