User.php

<?php

namespace DesignPatterns\Behavioral\Observer;

/**
 * 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
{
    /**
     * @var string
     */
    private $email;

    /**
     * @var \SplObjectStorage
     */
    private $observers;

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

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

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

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

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