WordPress at Your Fingertips
function is not described

_get_term_hierarchy() WP 2.3.0

Retrieves children of taxonomy as Term IDs.

This is an internal function for using it by WP core itself. It's not recommended to use this function in your code.

No Hooks.

Return

Array. Empty if $taxonomy isn't hierarchical or returns children as Term IDs.

Usage

_get_term_hierarchy( $taxonomy );
$taxonomy(string) (required)
Taxonomy name.

Changelog

Since 2.3.0 Introduced.

Code of _get_term_hierarchy() WP 5.8

<?php
function _get_term_hierarchy( $taxonomy ) {
	if ( ! is_taxonomy_hierarchical( $taxonomy ) ) {
		return array();
	}
	$children = get_option( "{$taxonomy}_children" );

	if ( is_array( $children ) ) {
		return $children;
	}
	$children = array();
	$terms    = get_terms(
		array(
			'taxonomy'               => $taxonomy,
			'get'                    => 'all',
			'orderby'                => 'id',
			'fields'                 => 'id=>parent',
			'update_term_meta_cache' => false,
		)
	);
	foreach ( $terms as $term_id => $parent ) {
		if ( $parent > 0 ) {
			$children[ $parent ][] = $term_id;
		}
	}
	update_option( "{$taxonomy}_children", $children );

	return $children;
}