function expunge()
    {
        if ($this->type == 'db' && $this->db && $this->ttl) {
            $this->db->query(
                "DELETE FROM {$this->table}"
                . " WHERE `cache_key` LIKE ?"
                . " AND `expires` < " . $this->db->now(),
                $this->prefix . '.%');
        }
    }