Without Demeter

EmailService.php

<?php declare(strict_types=1);

interface EmailService
{
    public function sendEmail(Email $email): void;
}

MySqlUserRepository.php

<?php declare(strict_types=1);

class MySqlUserRepository implements UserRepository
{
    public function save(User $user): void
    {
        // Implementation    
    }
    
    public function get(string $userId): User
    {
        // Implementation
    }
}

RegisterUserHandle.php

<?php declare(strict_types=1);

class RegisterUserHandler
{
    private $userRepository;
    private $notifications
    
    public function __construct(UserRepository $userRepository, WelcomeEmailSender $notificationService)
    {
        $this->userRepository = $userRepository;
        $this->notificationService = $notificationService;
    }
    
    public function handle($name, $email)
    {
        $user = new User($email, $name);
        $this->userRepository->save($user);
        $this->notificationService->send($user->getEmail(), $user->getName());
    }  
}

SmtpEmailService.php

<?php declare(strict_types=1);

class SmtpEmailService implements EmailService
{
    public function sendEmail(Email $email): void
    {
        // implementation
    }
}

UserRepository.php

<?php declare(strict_types=1);

interface UserRepository
{
    public function save(User $user): void;
    public function get(string $userId): User;
}

WelcomeEmailSender.php

<?php declare(strict_types=1);

final class WelcomeEmailSender
{
    private $emailService;
    
    public function __construct(EmailService $emailService)
    {
        $this->emailService = $emailService;
    }
    
    public function send(string $emailAddress, string $username): void
    {
        $body = $this->composeBody($username);
        $email = new Email($emailAddress, 'Welcome to our platform', $body);
       
    }
    
    private function composeBody(string $username)
    {
        // Code to compose body
    }
}