PHP中如何实现观察者模式？

<?php
interface Observer {
    public function update($data);
}

interface Subject {
    public function attach(Observer $observer);
    public function detach(Observer $observer);
    public function notify();
}

class ConcreteSubject implements Subject {
    private $observers = [];
    private $state;

    public function attach(Observer $observer) {
        $this->observers[] = $observer;
    }

    public function detach(Observer $observer) {
        $this->observers = array_filter($this->observers, function($obs) use ($observer) {
            return $obs !== $observer;
        });
    }

    public function notify() {
        foreach ($this->observers as $observer) {
            $observer->update($this->state);
        }
    }

    public function setState($state) {
        $this->state = $state;
        $this->notify();
    }
}

class ConcreteObserver implements Observer {
    private $name;

    public function __construct($name) {
        $this->name = $name;
    }

    public function update($data) {
        echo "Observer {$this->name} received update: {$data}\n";
    }
}

$subject = new ConcreteSubject();

$observer1 = new ConcreteObserver('Observer 1');
$observer2 = new ConcreteObserver('Observer 2');

$subject->attach($observer1);
$subject->attach($observer2);

$subject->setState('New State');
?>