Convert output of nav_menu items into a tree-like multidimensional array

Is there any way to grab the nav menu items as a multidimensional array instead of a flat array?

By a tree-like structure I mean something that would preserve relationship between child and parent items, like so (this is just an example)…

array(
  array(
    'post_type' => 'page',
    'post_name' => 'Home',
    'children' => array() 
  ),
  array(
    'post_type' => 'page',
    'post_name' => 'About Us',
    'children' => array(
      array(
        'post_type' => 'page',
        'post_name' => 'Our History',
        'children' => array() 
      )
    ) 
  )
)

There is a wp_get_nav_menu_items() function but it returns a 1-dimensional array with all the items on the same level, which is not what I want. Does WordPress include a built-in way to get a multidimensional array for my menu items? If not, what is the best way to get wp_get_nav_menu_items() as a tree-like structure into a multidimensional array in terms of performance?

Solutions Collecting From Web of "Convert output of nav_menu items into a tree-like multidimensional array"

The problem of building a tree from a flat array has been solved here with this, slightly modified, recursive solution:

/**
 * Modification of "Build a tree from a flat array in PHP"
 *
 * Authors: @DSkinner, @ImmortalFirefly and @SteveEdson
 *
 * @link https://stackoverflow.com/a/28429487/2078474
 */
function buildTree( array &$elements, $parentId = 0 )
{
    $branch = array();
    foreach ( $elements as &$element )
    {
        if ( $element->menu_item_parent == $parentId )
        {
            $children = buildTree( $elements, $element->ID );
            if ( $children )
                $element->wpse_children = $children;

            $branch[$element->ID] = $element;
            unset( $element );
        }
    }
    return $branch;
}

where we added the prefixed wpse_children attribute to avoid name collision.

Now we only have to define a simple helper function:

/**
 * Transform a navigational menu to it's tree structure
 *
 * @uses  buildTree()
 * @uses  wp_get_nav_menu_items()
 *
 * @param  String     $menud_id 
 * @return Array|null $tree 
 */
function wpse_nav_menu_2_tree( $menu_id )
{
    $items = wp_get_nav_menu_items( $menu_id );
    return  $items ? buildTree( $items, 0 ) : null;
}

Now it becomes super easy to transform a navigational menu into it’s tree structure with:

$tree = wpse_nav_menu_2_tree( 'my_menu' );  // <-- Modify this to your needs!
print_r( $tree );

For JSON, we can simply use:

$json = json_encode( $tree );

For a slightly different version, where we handpicked the attributes, check out the first revision of this answer here.

Update: Walker Class

Here’s a rather sketchy idea how we could try to hook into the recursive part of the the display_element() method of the abstract Walker class.

$w = new WPSE_Nav_Menu_Tree;
$args = (object) [ 'items_wrap' => '', 'depth' => 0, 'walker' => $w ];
$items = wp_get_nav_menu_items( 'my_menu' );
walk_nav_menu_tree( $items, $args->depth, $args );
print_r( $w->branch );  

where WPSE_Nav_Menu_Tree is an extension of the Walker_Nav_Menu class:

class WPSE_Nav_Menu_Tree extends Walker_Nav_Menu
{
   public $branch = [];

   public function display_element($element, &$children, $max_depth, $depth = 0, $args, &$output )
   {
      if( 0 == $depth )
         $this->branch[$element->ID] = $element;

      if ( isset($children[$element->ID] ) )
         $element->wpse_children = $children[$element->ID];

      parent::display_element($element, $children, $max_depth, $depth, $args, $output);
   }
}

This might give us an alternative approach if it works.

In short the function bellow does create the tree of objects by putting children into a new children property inside the parent object.

Code:

function wpse170033_nav_menu_object_tree( $nav_menu_items_array ) {
    foreach ( $nav_menu_items_array as $key => $value ) {
        $value->children = array();
        $nav_menu_items_array[ $key ] = $value;
    }

    $nav_menu_levels = array();
    $index = 0;
    if ( ! empty( $nav_menu_items_array ) ) do {
        if ( $index == 0 ) {
            foreach ( $nav_menu_items_array as $key => $obj ) {
                if ( $obj->menu_item_parent == 0 ) {
                    $nav_menu_levels[ $index ][] = $obj;
                    unset( $nav_menu_items_array[ $key ] );
                }
            }
        } else {
            foreach ( $nav_menu_items_array as $key => $obj ) {
                if ( in_array( $obj->menu_item_parent, $last_level_ids ) ) {
                    $nav_menu_levels[ $index ][] = $obj;
                    unset( $nav_menu_items_array[ $key ] );
                }
            }
        }
        $last_level_ids = wp_list_pluck( $nav_menu_levels[ $index ], 'db_id' );
        $index++;
    } while ( ! empty( $nav_menu_items_array ) );

    $nav_menu_levels_reverse = array_reverse( $nav_menu_levels );

    $nav_menu_tree_build = array();
    $index = 0;
    if ( ! empty( $nav_menu_levels_reverse ) ) do {
        if ( count( $nav_menu_levels_reverse ) == 1 ) {
            $nav_menu_tree_build = $nav_menu_levels_reverse;
        }
        $current_level = array_shift( $nav_menu_levels_reverse );
        if ( isset( $nav_menu_levels_reverse[ $index ] ) ) {
            $next_level = $nav_menu_levels_reverse[ $index ];
            foreach ( $next_level as $nkey => $nval ) {
                foreach ( $current_level as $ckey => $cval ) {
                    if ( $nval->db_id == $cval->menu_item_parent ) {
                        $nval->children[] = $cval;
                    }
                }
            }
        }
    } while ( ! empty( $nav_menu_levels_reverse ) );

    $nav_menu_object_tree = $nav_menu_tree_build[ 0 ];
    return $nav_menu_object_tree;
}

Usage:

$nav_menu_items = wp_get_nav_menu_items( 'main-menu' );
wpse170033_nav_menu_object_tree( $nav_menu_items );

Output:

Array
(
 [0] => WP_Post Object
  (
   [ID] => 51
   [post_author] => 1
   [post_date] => 2015-06-26 21:13:23
   [post_date_gmt] => 2015-06-26 19:13:23
   [post_content] => 
   [post_title] => 
   [post_excerpt] => 
   [post_status] => publish
   [comment_status] => open
   [ping_status] => open
   [post_password] => 
   [post_name] => 51
   [to_ping] => 
   [pinged] => 
   [post_modified] => 2015-07-29 20:55:10
   [post_modified_gmt] => 2015-07-29 18:55:10
   [post_content_filtered] => 
   [post_parent] => 0
   [guid] => http://example.com/?p=51
   [menu_order] => 1
   [post_type] => nav_menu_item
   [post_mime_type] => 
   [comment_count] => 0
   [filter] => raw
   [db_id] => 51
   [menu_item_parent] => 0
   [object_id] => 2
   [object] => page
   [type] => post_type
   [type_label] => Page
   [url] => http://example.com/example-page/
   [title] => Example-Page-1
   [target] => 
   [attr_title] => 
   [description] => 
   [classes] => Array
    (
     [0] => 
    )
   [xfn] => 
   [children] => Array
    (
     [0] => WP_Post Object
      (
       [ID] => 80
       [post_author] => 1
       [post_date] => 2015-06-27 14:03:31
       [post_date_gmt] => 2015-06-27 12:03:31
       [post_content] => 
       [post_title] => 
       [post_excerpt] => 
       [post_status] => publish
       [comment_status] => open
       [ping_status] => open
       [post_password] => 
       [post_name] => 80
       [to_ping] => 
       [pinged] => 
       [post_modified] => 2015-07-29 20:55:10
       [post_modified_gmt] => 2015-07-29 18:55:10
       [post_content_filtered] => 
       [post_parent] => 2
       [guid] => http://example.com/?p=80
       [menu_order] => 2
       [post_type] => nav_menu_item
       [post_mime_type] => 
       [comment_count] => 0
       [filter] => raw
       [db_id] => 80
       [menu_item_parent] => 51
       [object_id] => 69
       [object] => page
       [type] => post_type
       [type_label] => Page
       [url] => http://example.com/example-page/subpage-1/
       [title] => Subpage-1
       [target] => 
       [attr_title] => 
       [description] => 
       [classes] => Array
        (
         [0] => 
        )
       [xfn] => 
       [children] => Array
        (
        )
      )
    )
  )
 [1] => WP_Post Object
  (
   [ID] => 49
   [post_author] => 1
   [post_date] => 2015-06-26 21:13:23
   [post_date_gmt] => 2015-06-26 19:13:23
   [post_content] => 
   [post_title] => 
   [post_excerpt] => 
   [post_status] => publish
   [comment_status] => open
   [ping_status] => open
   [post_password] => 
   [post_name] => 49
   [to_ping] => 
   [pinged] => 
   [post_modified] => 2015-07-29 20:55:10
   [post_modified_gmt] => 2015-07-29 18:55:10
   [post_content_filtered] => 
   [post_parent] => 0
   [guid] => http://example.com/?p=49
   [menu_order] => 3
   [post_type] => nav_menu_item
   [post_mime_type] => 
   [comment_count] => 0
   [filter] => raw
   [db_id] => 49
   [menu_item_parent] => 0
   [object_id] => 41
   [object] => page
   [type] => post_type
   [type_label] => Page
   [url] => http://example.com/example-page-2/
   [title] => Example-Page-2
   [target] => 
   [attr_title] => 
   [description] => 
   [classes] => Array
    (
     [0] => 
    )
   [xfn] => 
   [children] => Array
    (
    )
  )
 [2] => WP_Post Object
  (
   [ID] => 48
   [post_author] => 1
   [post_date] => 2015-06-26 21:13:23
   [post_date_gmt] => 2015-06-26 19:13:23
   [post_content] => 
   [post_title] => 
   [post_excerpt] => 
   [post_status] => publish
   [comment_status] => open
   [ping_status] => open
   [post_password] => 
   [post_name] => 48
   [to_ping] => 
   [pinged] => 
   [post_modified] => 2015-07-29 20:55:10
   [post_modified_gmt] => 2015-07-29 18:55:10
   [post_content_filtered] => 
   [post_parent] => 0
   [guid] => http://example.com/?p=48
   [menu_order] => 4
   [post_type] => nav_menu_item
   [post_mime_type] => 
   [comment_count] => 0
   [filter] => raw
   [db_id] => 48
   [menu_item_parent] => 0
   [object_id] => 42
   [object] => page
   [type] => post_type
   [type_label] => Page
   [url] => http://example.com/example-page-3/
   [title] => Example-Page-3
   [target] => 
   [attr_title] => 
   [description] => 
   [classes] => Array
    (
     [0] => 
    )
   [xfn] => 
   [children] => Array
    (
     [0] => WP_Post Object
      (
       [ID] => 79
       [post_author] => 1
       [post_date] => 2015-06-27 14:03:31
       [post_date_gmt] => 2015-06-27 12:03:31
       [post_content] => 
       [post_title] => 
       [post_excerpt] => 
       [post_status] => publish
       [comment_status] => open
       [ping_status] => open
       [post_password] => 
       [post_name] => 79
       [to_ping] => 
       [pinged] => 
       [post_modified] => 2015-07-29 20:55:10
       [post_modified_gmt] => 2015-07-29 18:55:10
       [post_content_filtered] => 
       [post_parent] => 42
       [guid] => http://example.com/?p=79
       [menu_order] => 5
       [post_type] => nav_menu_item
       [post_mime_type] => 
       [comment_count] => 0
       [filter] => raw
       [db_id] => 79
       [menu_item_parent] => 48
       [object_id] => 70
       [object] => page
       [type] => post_type
       [type_label] => Page
       [url] => http://example.com/example-page-3/subpage-2/
       [title] => Subpage-2
       [target] => 
       [attr_title] => 
       [description] => 
       [classes] => Array
        (
         [0] => 
        )
       [xfn] => 
       [children] => Array
        (
         [0] => WP_Post Object
          (
           [ID] => 78
           [post_author] => 1
           [post_date] => 2015-06-27 14:03:31
           [post_date_gmt] => 2015-06-27 12:03:31
           [post_content] => 
           [post_title] => 
           [post_excerpt] => 
           [post_status] => publish
           [comment_status] => open
           [ping_status] => open
           [post_password] => 
           [post_name] => 78
           [to_ping] => 
           [pinged] => 
           [post_modified] => 2015-07-29 20:55:10
           [post_modified_gmt] => 2015-07-29 18:55:10
           [post_content_filtered] => 
           [post_parent] => 70
           [guid] => http://example.com/?p=78
           [menu_order] => 6
           [post_type] => nav_menu_item
           [post_mime_type] => 
           [comment_count] => 0
           [filter] => raw
           [db_id] => 78
           [menu_item_parent] => 79
           [object_id] => 76
           [object] => page
           [type] => post_type
           [type_label] => Page
           [url] => http://example.com/example-page-3/subpage-2/subpage-3/
           [title] => Subpage-3
           [target] => 
           [attr_title] => 
           [description] => 
           [classes] => Array
            (
             [0] => 
            )
           [xfn] => 
           [children] => Array
            (
             [0] => WP_Post Object
              (
               [ID] => 87
               [post_author] => 1
               [post_date] => 2015-06-27 15:27:08
               [post_date_gmt] => 2015-06-27 13:27:08
               [post_content] => 
               [post_title] => 
               [post_excerpt] => 
               [post_status] => publish
               [comment_status] => open
               [ping_status] => open
               [post_password] => 
               [post_name] => 87
               [to_ping] => 
               [pinged] => 
               [post_modified] => 2015-07-29 20:55:10
               [post_modified_gmt] => 2015-07-29 18:55:10
               [post_content_filtered] => 
               [post_parent] => 76
               [guid] => http://example.com/?p=87
               [menu_order] => 7
               [post_type] => nav_menu_item
               [post_mime_type] => 
               [comment_count] => 0
               [filter] => raw
               [db_id] => 87
               [menu_item_parent] => 78
               [object_id] => 85
               [object] => page
               [type] => post_type
               [type_label] => Page
               [url] => http://example.com/example-page-3/subpage-2/subpage-3/subpage-4/
               [title] => Subpage-4
               [target] => 
               [attr_title] => 
               [description] => 
               [classes] => Array
                (
                 [0] => 
                )
               [xfn] => 
               [children] => Array
                (
                 [0] => WP_Post Object
                  (
                   [ID] => 366
                   [post_author] => 1
                   [post_date] => 2015-07-29 20:52:46
                   [post_date_gmt] => 2015-07-29 18:52:46
                   [post_content] => 
                   [post_title] => 
                   [post_excerpt] => 
                   [post_status] => publish
                   [comment_status] => open
                   [ping_status] => open
                   [post_password] => 
                   [post_name] => 366
                   [to_ping] => 
                   [pinged] => 
                   [post_modified] => 2015-07-29 20:55:10
                   [post_modified_gmt] => 2015-07-29 18:55:10
                   [post_content_filtered] => 
                   [post_parent] => 85
                   [guid] => http://example.com/?p=366
                   [menu_order] => 8
                   [post_type] => nav_menu_item
                   [post_mime_type] => 
                   [comment_count] => 0
                   [filter] => raw
                   [db_id] => 366
                   [menu_item_parent] => 87
                   [object_id] => 112
                   [object] => page
                   [type] => post_type
                   [type_label] => Page
                   [url] => http://example.com/example-page-3/subpage-2/subpage-3/subpage-4/subpage-5/
                   [title] => Subpage-5
                   [target] => 
                   [attr_title] => 
                   [description] => 
                   [classes] => Array
                    (
                     [0] => 
                    )
                   [xfn] => 
                   [children] => Array
                    (
                    )
                  )
                )
              )
            )
          )
        )
      )
    )
  )
)

There may be a misunderstanding here about WordPress nav menu items as tree-like structures.

Tree-like structures in WordPress ARE NOT MULTIDIMENSIONAL ARRAYS!

Note that while the returned menu items array is flat, it is still a tree-like structure because each item contains information about it’s parentage (the parent value is either 0 if the item has no parent – or the id of the parent item if it does).

When you pass such an array to the Walker class, it loops through the results and creates two arrays – one containing top-level items, and another containing child items in format $parent_id => array() where the array contains menu items that are direct children of that element.

The walker then loops through the top level items array, processing that item and then checking the children array to see if there are any children for the current element, and processes each of those the same way, recursively.

If you want to know how to convert a WordPress tree-like structure into a multidimensional array, that is a different question entirely (and not technically a WordPress question). But the information returned by wp_get_nav_menu_items() is a tree-like structure… and you can use Walker as-is to handle it.

If you want to see the exact code that WordPress’s Walker class runs to walk the array, take a look at Walker->walk() on WordPress Trac from lines 213-258. You could use that code as-is to build a multidimensional array, if that’s what you’re after.

Walkers

WordPress is designed to use the Walker class to process it’s tree-like structures. If you are simply rendering a menu, or really just need the ultimate output, you may want to consider using wp_nav_menu() to output your menu…

Example:

wp_nav_menu(array(
    'menu' => 6, // your menu id, name, or slug
    'echo' => true, // set this to false if you want a string back instead
    'walker' => new Your_Walker(),
));

You would extend the Walker class (e.g. Your_Walker()) to get whatever output you need. For an example, see this entry on the Codex.