bbp_forum_query_last_reply_id( int $forum_id, int $topic_ids )

function bbp_forum_query_last_reply_id( $forum_id, $topic_ids = 0 ) {
	global $wpdb;

	$cache_id = 'bbp_get_forum_' . $forum_id . '_reply_id';
	$reply_id = wp_cache_get( $cache_id, 'bbpress_posts' );

	if ( false === $reply_id ) {

		if ( empty( $topic_ids ) ) {
			$topic_ids = bbp_forum_query_topic_ids( $forum_id );
		}

		if ( !empty( $topic_ids ) ) {
			$topic_ids = implode( ',', wp_parse_id_list( $topic_ids ) );
			$reply_id  = (int) $wpdb->get_var( $wpdb->prepare( "SELECT ID FROM {$wpdb->posts} WHERE post_parent IN ( {$topic_ids} ) AND post_status = '%s' AND post_type = '%s' ORDER BY ID DESC LIMIT 1;", bbp_get_public_status_id(), bbp_get_reply_post_type() ) );
			wp_cache_set( $cache_id, $reply_id, 'bbpress_posts' ); // May be (int) 0
		} else {
			wp_cache_set( $cache_id, '0', 'bbpress_posts' );
		}
	}

	return (int) apply_filters( 'bbp_get_forum_last_reply_id', (int) $reply_id, $forum_id );
}