bbp_update_forum_last_topic_id( int $forum_id, int $topic_id )

function bbp_update_forum_last_topic_id( $forum_id = 0, $topic_id = 0 ) {
	$forum_id = bbp_get_forum_id( $forum_id );

	// Define local variable(s)
	$children_last_topic = 0;

	// Do some calculation if not manually set
	if ( empty( $topic_id ) ) {

		// Loop through children and add together forum reply counts
		$children = bbp_forum_query_subforum_ids( $forum_id );
		if ( !empty( $children ) ) {
			foreach ( (array) $children as $child ) {
				$children_last_topic = bbp_update_forum_last_topic_id( $child ); // Recursive
			}
		}

		// Setup recent topic query vars
		$post_vars = array(
			'post_parent' => $forum_id,
			'post_type'   => bbp_get_topic_post_type(),
			'meta_key'    => '_bbp_last_active_time',
			'orderby'     => 'meta_value',
			'numberposts' => 1
		);

		// Get the most recent topic in this forum_id
		$recent_topic = get_posts( $post_vars );
		if ( !empty( $recent_topic ) ) {
			$topic_id = $recent_topic[0]->ID;
		}
	}

	// Cast as integer in case of empty or string
	$topic_id            = (int) $topic_id;
	$children_last_topic = (int) $children_last_topic;

	// If child forums have higher id, use that instead
	if ( !empty( $children ) && ( $children_last_topic > $topic_id ) )
		$topic_id = $children_last_topic;

	// Update the last public topic ID
	if ( bbp_is_topic_published( $topic_id ) )
		update_post_meta( $forum_id, '_bbp_last_topic_id', $topic_id );

	return (int) apply_filters( 'bbp_update_forum_last_topic_id', $topic_id, $forum_id );
}