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_get_term_children() WP 2.3.0

Get the subset of $terms that are descendants of $term_id.

If $terms is an array of objects, then _get_term_children() returns an array of objects. If $terms is an array of IDs, then _get_term_children() returns an array of IDs.

No Hooks.

Return

Array/WP_Error. The subset of $terms that are descendants of $term_id.

Usage

_get_term_children( $term_id, $terms, $taxonomy, &$ancestors );
$term_id(int) (required)
The ancestor term: all returned terms should be descendants of $term_id.
$terms(array) (required)
The set of terms - either an array of term objects or term IDs - from which those that are descendants of $term_id will be chosen.
$taxonomy(string) (required)
The taxonomy which determines the hierarchy of the terms.
$ancestors(array) (required)
Term ancestors that have already been identified. Passed by reference, to keep track of found terms when recursing the hierarchy. The array of located ancestors is used to prevent infinite recursion loops. For performance, term_ids are used as array keys, with 1 as value.
Default: empty array

Code of get term children: wp-includes/taxonomy.php VER 5.1.1

<?php
function _get_term_children( $term_id, $terms, $taxonomy, &$ancestors = array() ) {
	$empty_array = array();
	if ( empty( $terms ) ) {
		return $empty_array;
	}

	$term_list    = array();
	$has_children = _get_term_hierarchy( $taxonomy );

	if ( ( 0 != $term_id ) && ! isset( $has_children[ $term_id ] ) ) {
		return $empty_array;
	}

	// Include the term itself in the ancestors array, so we can properly detect when a loop has occurred.
	if ( empty( $ancestors ) ) {
		$ancestors[ $term_id ] = 1;
	}

	foreach ( (array) $terms as $term ) {
		$use_id = false;
		if ( ! is_object( $term ) ) {
			$term = get_term( $term, $taxonomy );
			if ( is_wp_error( $term ) ) {
				return $term;
			}
			$use_id = true;
		}

		// Don't recurse if we've already identified the term as a child - this indicates a loop.
		if ( isset( $ancestors[ $term->term_id ] ) ) {
			continue;
		}

		if ( $term->parent == $term_id ) {
			if ( $use_id ) {
				$term_list[] = $term->term_id;
			} else {
				$term_list[] = $term;
			}

			if ( ! isset( $has_children[ $term->term_id ] ) ) {
				continue;
			}

			$ancestors[ $term->term_id ] = 1;

			if ( $children = _get_term_children( $term->term_id, $terms, $taxonomy, $ancestors ) ) {
				$term_list = array_merge( $term_list, $children );
			}
		}
	}

	return $term_list;
}