service-layer.php

<?php

class Email
{
    private string $email;

    public function __construct(string $email)
    {
        if (false === filter_var($email, FILTER_VALIDATE_EMAIL)) {
            throw new Exception('Invalid Email');
        }

        $this->email = $email;
    }

    public function getEmail(): string { ... }
}

class Password
{
    public function __construct(string $password) { ... }
}

class RegisterUser
{
    public function __construct(string $name, Email $email, Password $password) { ... }
}

class RegisterUserService
{
    public function register(RegisterUser $registerUser): UserRegistered  { ... }
}