public function add_data() {
        $query = "INSERT INTO users(id, name, level, age) VALUES (null, :name, :level, :age)";

        $name  = $_REQUEST['name'];
        $level = $_REQUEST['level'];
        $age   = $_REQUEST['age'];

        $params = array(':name' => $this->name,
                        ':level'=> $this->level,
                        ':age'  => $this->age);

        $stmt = $this->pdo->prepare($query);
        $stmt->execute($params);
    }