Ejemplo de patron observer

<?php
abstract class Observable{
    protected $observers;
    function __construct(){
        $this->observers = array();
    }

    public function registrarObserver($observer){
        if(!in_array($observer, $this->observers)){
            $this->observers[] = $observer;
        }
    }

    public function deregistrarObserver($observer){
        if(in_array($observer, $this->observers)){
            $key = array_search($observer, $this->observers);
            unset($this->observers[$key]);
        }
    }

    abstract public function notificarObservers();

}

interface Observer{
    public function notificar($sender, $params);
}

class MiObservable extends Observable{

    public function __construct(){
        parent::__construct();
    }

    public function notificarObservers(){
        foreach ($this->observers as $observer) {
            $observer->notificar($this, $this->param);
        }
    }

    public function Evento($texto){
        $this->param = $texto;
        $this->notificarObservers();
    }
}

class Log implements Observer{
    public function notificar($sender, $param){
        echo get_class($sender)." envio $param a las ".date('h:i:s', time())."<br />";
    }
}

class SalvarLog implements Observer{
    public function notificar($sender, $param){
        echo "Guardando en BD $param enviado por ".get_class($sender)."... <br /><br />";
    }

}

$obj = new MiObservable();
$obj->registrarObserver(new Log());
$obj->registrarObserver(new SalvarLog());

$obj->Evento('Test 1');
sleep(1);
$obj->Evento('Test 2');

$obj->deregistrarObserver(new SalvarLog());
$obj->Evento('Test 3');
  ?>