WordPress: Get All Descendants

get-all-descendants.php

<?php

/**
 * Gets all of the descendant/child post IDs for a parent post ID.
 *
 * @param  int $parent_post_id The parent post ID.
 * @return array
 */
function get_all_descendants( $parent_post_id ) {

	$post = get_post( $parent_post_id );
	if ( ! is_a( $post, '\WP_Post' ) ) {
		return false;
	}

	$child_post_ids = [];

	// Get children for this parent.
	$children = get_children(
		[
			'post_parent' => $parent_post_id,
			'post_type'   => get_post_type( $parent_post_id ),
		]
	);

	foreach ( $children as $child_post ) {

		$child_post_ids[] = $child_post->ID;

		// Recurse back into this.
		$child_ids = get_all_descendants( $child_post->ID );

		if ( ! empty( $child_ids ) ) {
			$child_post_ids = array_merge( $child_post_ids, $child_ids );
		}
	}

	return $child_post_ids;
}