<?php

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 string $email;
    private SplObjectStorage $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);
        }
    }
}


use SplObserver;
use SplSubject;

class UserObserver implements SplObserver
{
    /**
     * @var SplSubject[]
     */
    private array $changedUsers = [];

    /**
     * It is called by the Subject, usually by SplSubject::notify()
     */
    public function update(SplSubject $subject)
    {
        $this->changedUsers[] = clone $subject;
    }

    /**
     * @return SplSubject[]
     */
    public function getChangedUsers(): array
    {
        return $this->changedUsers;
    }
}

// USAGE
  $observer = new UserObserver();
  $user = new User();
  
  $user->attach($observer);
  $user->changeEmail('foo@bar.com');
  $this->assertCount(1, $observer->getChangedUsers());