class Cart
{
    private $_items
    private $_listeners;

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

    public function add($cd)
    {
        $this->_items[$cd] = (isset($this->_items[$cd]))? ++$this->_items[$cd] : 1;
        $this->notify();
    }

    public function remove($cd)
    {
        $this->_items[$cd] = (isset($this->_items[$cd]))? --$this->_items[$cd] : 0;
        if($this->_items[$cd] <= 0){
            unset($this->_items[$cd]);
        }
        $this->notify();
    }

    public function get()
    {
        return $this->_items;
    }

    public function has($cd)
    {
        return array_key_exists($cd, $this->_items);
    }

    public function notify()
    {
        foreach($this->_listeners as $listener){
            $listener->update($this);
        }
    }

    public function addListener(CartListener $listener)
    {
        $this->_listeners[get_class($listener)] = $listener;
    }
}

Listener.php

以下のようにインターフェースを定義する。

interface Listener
{
    public function update(Cart $cart);
}

class PresentListener implements Listener
{
    const PRESENT_TARGET = '30:cookie';
    const PRESENT_ITEM   = '99:present';

    public function __construct()
    {
    }

    public function update(Cart $cart)
    {
        if($cart->hasItem(self::PRESENT_TARGET) && !$cart->hasItem(self::PRESENT_ITEM)){
            $cart->add(self::PRESENT_ITEM);
        }
        if(!$cart->hasItem(self::PRESENT_TARGET) && $cart->hasItem(self::PRESENT_ITEM)){
            $cart->remove(self::PRESENT_ITEM);
        }
    }
}

class LoginListener implements Listener
{
    public function __construct()
    {
    }

    public function update(Cart $cart)
    {
        var_dump($cart->get());
    }
}