class User
{
    public function setLogin($login)
    {
       $this->login = $login;
    }

    public function setPassword($password)
    {
       $this->password = $password;
    }

    public function registration()
    {
       // Регистрация
    }
    
    public function enter()
    {
       // Вход
    }    
}


class UserRegistrationService 
{
    public function __construct($login, $password, $confirm)
    {
       $this->login    = $login;
       $this->password = $password;
       $this->confirm  = $confirm;
       $this->user = new User;
    }

    public function registration($checker)
    {
       $this->setLogin($checker);
       $this->setPassword();
       $this->user->registration();
       return $this->user;  
    }  

    protected function setLogin($checker)
    {
        if (!$checker->checkLogin($this->login)) {
           throw new Exception();
        }
        
        $this->user->setLogin($this->login);  
    }
    
    protected function setPassword()
    {
        if ($this->password !== $this->confirm) {
           throw new Exception();
        }
        
        $this->user->setPassword($password); 
    }     
}

$service = new UserRegistrationService('login', 'password', 'password');
$service->registration(new UsernameAlreadyRegisteredChecker);