A collection of helper functions for terms.

terms.php

/**
 * Returns the terms associated with the given object, hierarchically structured.
 *
 * @param  array           $object_id  The ID of the object to retrieve.
 * @param  string|string[] $taxonomies The taxonomy names to retrieve terms from.
 * @return array
 */
function get_object_hierarchical_terms( $object_id, $taxonomies ) {

	// Retrieve the terms.
	$terms = wp_get_object_terms( $object_id, $taxonomies );

	$hierarchical_terms = [];
	foreach ( $terms as $term ) {

		if ( $term->parent > 0 ) {
			continue;
		}

		$term_id                        = $term->term_id;
		$hierarchical_terms[ $term_id ] = $term;

		$child_terms = collect_terms( $term_id, $terms );
		if ( ! empty( $child_terms ) ) {
			$hierarchical_terms[ $term_id ]->children = $child_terms;
		}
	}

	return $hierarchical_terms;
}

/**
 * Collects an array of terms into an organised collection.
 *
 * @param  int   $parent_id The parent term ID.
 * @param  array $terms     Array of terms.
 * @return array
 */
function collect_terms( $parent_id, $terms ) {

	$children = [];
	foreach ( $terms as $term ) {

		if ( $term->parent !== $parent_id ) {
			continue;
		}

		$term_id              = $term->term_id;
		$children[ $term_id ] = $term;

		$child_terms = collect_terms( $term_id, $terms );
		if ( ! empty( $child_terms ) ) {
			$children[ $term_id ]->children = $child_terms;
		}
	}

	return $children;
}