Getting started with php observer pattern example

class DemoSubject implements SplSubject{
    private $observers, $value;

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

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

    public function detach(SplObserver $observer){
        if($idx = array_search($observer, $this->observers, true)){
            unset($this->observers[$idx]);
        }
    }

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

    public function setValue($value){
        $this->value = $value;
        $this->notify();
    }

    public function getValue(){
        return $this->value;
    }
}

class DemoObserver implements SplObserver{
    public function update(SplSubject $subject){
        echo 'The new value is '. $subject->getValue();
    }
}

$subject = new DemoSubject();
$observer = new DemoObserver();
$subject->attach($observer);
$subject->setValue(5);