public function login($data)
    {
        $sql = 'SELECT * FROM users WHERE username=:username AND password=:password';
        $query = database::connect()
            ->prepare($sql);
        $query->execute(array(':username' => $data['username'],':password' =>$data['password']));

        $user = $query->fetch(PDO::FETCH_ASSOC);
        if($user)
        {
            $_SESSION['logged'] = true;
            $_SESSION['username']= $user['username'];
            $_SESSION['role'] = $user['role'];
        }
        database::disconnect();
        header('location:index.php');
    }