get_term_children() │ WP 2.3.0

Merge all term children into a single array of their IDs.

This recursive function will merge all of the children of $term into the same array of term IDs. Only useful for taxonomies which are hierarchical.

Will return an empty array if $term does not exist in $taxonomy.

Uses: _get_term_hierarchy()
1 time — 0.000048 sec (very fast) | 50000 times — 0.75 sec (very fast) | PHP 7.0.5, WP 4.5.2

No Hooks.

Return

Array|WP_Error. List of Term IDs. WP_Error returned if $taxonomy does not exist.

Usage

get_term_children( $term_id, $taxonomy );
$term_id(int) (required)
ID of term to get children.
$taxonomy(string) (required)
Taxonomy name.

Changelog

Since 2.3.0 Introduced.

Code of get_term_children() WP 5.9.3

wp-includes/taxonomy.php 
function get_term_children( $term_id, $taxonomy ) {
	if ( ! taxonomy_exists( $taxonomy ) ) {
		return new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy.' ) );
	}

	$term_id = (int) $term_id;

	$terms = _get_term_hierarchy( $taxonomy );

	if ( ! isset( $terms[ $term_id ] ) ) {
		return array();
	}

	$children = $terms[ $term_id ];

	foreach ( (array) $terms[ $term_id ] as $child ) {
		if ( $term_id === $child ) {
			continue;
		}

		if ( isset( $terms[ $child ] ) ) {
			$children = array_merge( $children, get_term_children( $child, $taxonomy ) );
		}
	}

	return $children;
}

Relations (of posts terms...)

Term (taxonomies terms)

Taxonomies: categories, tags, ...