public function checkAccountExist(User $user) {
        $prepare = $this->connexion->prepare('SELECT * FROM users WHERE username=:username AND password=:password');
        $prepare->execute(array(
            "username" => $user->getUsername(),
            "password" => $user->getPassword()
        ));
        $result = $prepare->fetchAll(PDO::FETCH_ASSOC);
        if(empty($result)==false) {
            return true;
        }
        return false;
    }