User.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Behavioral\Observer;

use SplSubject;
use SplObjectStorage;
use SplObserver;

/**
 * User implements the observed object (called Subject), it maintains a list of observers and sends notifications to
 * them in case changes are made on the User object
 */
class User implements SplSubject
{
    private SplObjectStorage $observers;
    private $email;

    public function __construct()
    {
        $this->observers = new SplObjectStorage();
    }

    public function attach(SplObserver $observer): void
    {
        $this->observers->attach($observer);
    }

    public function detach(SplObserver $observer): void
    {
        $this->observers->detach($observer);
    }

    public function changeEmail(string $email): void
    {
        $this->email = $email;
        $this->notify();
    }

    public function notify(): void
    {
        /** @var SplObserver $observer */
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }
}