$query = "";
        if($current_url!="") {
            $query = "UPDATE tb_sessions SET timestamp_last_activity = " . time() . ",url_last_activity = '" . $current_url . "' WHERE id_user=$user_id";
        }
        else{
            $query = "UPDATE tb_sessions SET timestamp_last_activity = " . time() . " WHERE id_user=$user_id";
        }
        $this->last_query = $query;
        $this->mysqli->query($query);