bbp_update_forum_reply_count( int $forum_id )

function bbp_update_forum_reply_count( $forum_id = 0 ) {
	global $wpdb;

	$forum_id = bbp_get_forum_id( $forum_id );
	$children_reply_count = 0;

	// 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_reply_count += bbp_update_forum_reply_count( $child );
		}
	}

	// Don't count replies if the forum is a category
	$topic_ids = bbp_forum_query_topic_ids( $forum_id );
	if ( !empty( $topic_ids ) ) {
		$topic_ids   = implode( ',', wp_parse_id_list( $topic_ids ) );
		$reply_count = (int) $wpdb->get_var( $wpdb->prepare( "SELECT COUNT(ID) FROM {$wpdb->posts} WHERE post_parent IN ( {$topic_ids} ) AND post_status = '%s' AND post_type = '%s';", bbp_get_public_status_id(), bbp_get_reply_post_type() ) );
	} else {
		$reply_count = 0;
	}

	// Calculate total replies in this forum
	$total_replies = (int) $reply_count + $children_reply_count;

	// Update the count
	update_post_meta( $forum_id, '_bbp_reply_count',       (int) $reply_count   );
	update_post_meta( $forum_id, '_bbp_total_reply_count', (int) $total_replies );

	return (int) apply_filters( 'bbp_update_forum_reply_count', (int) $total_replies, $forum_id );
}