カテゴリIDを階層順に並べた配列をつくる

functions.php

<?php
function get_categories_tree() {

    $post_categories = get_the_category();
    $cat_trees = array();
    $cat_counts = array();
    $cat_depth_max = 10;

    foreach ( $post_categories as $post_category ) {
        $depth = 0;
        $cat_IDs = array($post_category->cat_ID);
        $cat_obj = $post_category;

        while ( $depth < $cat_depth_max ) {
            if ( $cat_obj->category_parent == 0 ) {
                break;
            }
            $cat_obj = get_category($cat_obj->category_parent);
            array_unshift($cat_IDs, $cat_obj->cat_ID);
            $depth++;
        }
        array_push($cat_trees, $cat_IDs);
        array_push($cat_counts, count($cat_IDs));
    }

    $depth_max = max($cat_counts);
    $cat_key = array_search($depth_max, $cat_counts);
    $cat_tree = $cat_trees[$cat_key];
    return $cat_tree;
}