Reproducing hierarchical list output from wp_list_categories(), using get_categories()

Question

I need more control over the category listing output, so I'm using get_categories(), instead of wp_list_categories().

This function returns a flat array of objects ordered by a certain attribute. How can I build a hierarchical list from it like wp_list_categories() does?

Answer 1

The most ideal solution for walking over the data is to use the WordPress walker class.

http://codex.wordpress.org/Function_Reference/Walker_Class

You won't find many an example around the web for using it, but one was given by Scribu here.

http://scribu.net/wordpress/extending-the-category-walker.html

You can also look to the classes WordPress uses to extend the walker as further examples.

http://core.trac.wordpress.org/browser/tags/3.0.1/wp-includes/classes.php

Hope that helps..

Answer 2

How to build a hierarchical tree from get_categories

See this Stack Overflow question. In the answer Jan Fabry does it like this:

$categories = get_categories();

// First index all categories by parent id, for easy lookup later
$cats_by_parent = array();
foreach ($categories as $cat)
{
  $parent_id = $cat->category_parent;
  if (!array_key_exists($parent_id, $cats_by_parent)) 
  {
    $cats_by_parent[$parent_id] = array();
  }
  $cats_by_parent[$parent_id][] = $cat;
}

// Then build a hierarchical tree
$cat_tree = array();
function add_cats_to_bag(&$child_bag, &$children)
{
   global $cats_by_parent;
   foreach ($children as $child_cat) 
   {
     $child_id = $child_cat->cat_ID;
     if (array_key_exists($child_id, $cats_by_parent)) 
     {
        $child_cat->children = array();
        add_cats_to_bag($child_cat->children, $cats_by_parent[$child_id]);
     }
     $child_bag[$child_id] = $child_cat;
   }
}
add_cats_to_bag($cat_tree, $cats_by_parent[0]);