Get the post categories preserving the hierarchical order (WordPress)

get_post_categories.php

<?php
/**
 * Get the post categories preserving the hierarchical order
 *
 * This function returns an array with the following structure
 *
 * [
 *      [parent_term_id] => [
 *          [children] => [
 *              [child_term_id] => [
 *                  [children] => [
 *                      [...]
 *                  ],
 *                  [name]     => 'child_term_name',
 *              ],
 *          ],
 *	    [name] => 'parent_term_name
 *      ]
 *      [...]
 * ]
 *
 * @author Ramon Ahnert <https://profiles.wordpress.org/rahmohn/>, Felipe Elia <https://profiles.wordpress.org/felipeelia/>
 * @param int $post_id The post ID
 * @return array
 */
function get_post_categories( $post_id ) {
	$categories    = wp_list_sort( get_the_category( $post_id ), 'term_id' );
	$transient_key = 'post-' . $post_id . '-categories-tree';

	if ( empty( $categories ) ) {
		delete_transient( $transient_key );

		return [];
	}

	/**
	 * Return an array with term_id, name and parent
	 *
	 * @param WP_Term $category
	 * @return array
	 */
	$map_termid_name_parent = function ( $category ) {
		return [
			'term_id' => $category->term_id,
			'name'    => $category->name,
			'parent'  => $category->parent,
		];
	};

	$cached_categories = get_transient( $transient_key );

	// If categories don't changed, we'll return a cached value.
	if (
		false !== $cached_categories &&
		is_array( $cached_categories ) &&
		! empty( $cached_categories['categories_tree'] ) &&
		! empty( $cached_categories['term_id_name_parent_json'] )
	) {
		// We rely on term_id, name and parent to check if categories have changed
		$categories_termid_name_parent_json = wp_json_encode( array_map( $map_termid_name_parent, $categories ) );

		if ( $categories_termid_name_parent_json === $cached_categories['term_id_name_parent_json'] ) {
			return $cached_categories['categories_tree'];
		}
	}

	/**
	 * Build the category tree
	 *
	 * This function returns an array with the following structure
	 *
	 * [
	 *      [parent_term_id] => [
	 *          [children] => [
	 *              [child_term_id] => [
	 *                  [name]     => 'child_term_name',
	 *                  [children] => [
	 *                      [...]
	 *                  ]
	 *              ]
	 *          ]
	 *      ]
	 *      [...]
	 * ]
	 *
	 * @param array   $categories_tree  Array used to hold the categories (passed by reference)
	 * @param WP_Term $child_category   Child category
	 * @param array   $ancestors        The ancestors ids of child category from highest to lowest in the hierarchy
	 * @param integer $key              Position in the hierarchy of ancestors
	 * @param integer $depth_level      Check how depth we are to avoid an infinite loop
	 * @return void
	 */
	$build_category_tree = function ( &$categories_tree, $child_category, $ancestors, $key = 0, $depth_level = 0 ) use ( &$build_category_tree ) {
		$count = count( $ancestors );

		if ( $key < $count && 200 > $depth_level ) {
			// If the category is not on the category tree, add it.
			if ( ! array_key_exists( $ancestors[ $key ], $categories_tree ) ) {
				$categories_tree[ $ancestors[ $key ] ]['children'] = [];
			}

			// Fill with child category name
			if ( $ancestors[ $key ] === $child_category->parent ) {
				$categories_tree[ $ancestors[ $key ] ]['children'][ $child_category->term_id ]['name'] = $child_category->name;
			}

			// Go to the next level of hierarchy
			$build_category_tree( $categories_tree[ $ancestors[ $key ] ]['children'], $child_category, $ancestors, $key + 1, $depth_level + 1 );
		}

	};

	$categories_tree = [];
	// Build the category tree with all levels
	foreach ( $categories as $category ) {
		// Fill the category tree with the information of root categories
		if ( 0 === $category->parent ) {
			$categories_tree[ $category->term_id ]['name'] = $category->name;
			continue;
		}

		$ancestors = array_reverse( get_ancestors( $category->term_id, 'category', 'taxonomy' ) );

		$build_category_tree( $categories_tree, $category, $ancestors );
	}

	// Rely on term_id, name and parent to verify if categories have changed
	$categories_termid_name_parent = array_map( $map_termid_name_parent, $categories );

	set_transient(
		$transient_key,
		[
			'term_id_name_parent_json' => wp_json_encode( $categories_termid_name_parent ),
			'categories_tree'          => $categories_tree,
		]
	);

	return $categories_tree;
}

// print_r( get_post_categories( $YOUR_POST_ID ) );