Sample of observer pattern using PHP5.4 trait.

gistfile1.php

trait Subject
{
	public function initListeners()
	{
		$this->listeners = array();
	}

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

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

class Cart
{
	use Subject;

	public function addItem($item){
		$this->notify();
	}
}

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

class CartListener1 implements Listener
{
	public function update($cart)
	{
		echo "update 1!" . PHP_EOL;
	}
}

class CartListener2 implements Listener
{
	public function update($cart)
	{
		echo "update 2!!" . PHP_EOL;
	}
}

$cart = new Cart();
$cart->initListeners();
$cart->addListener(new CartListener1());
$cart->addListener(new CartListener2());

$cart->addItem('item');
/*
 update 1!
 update 2!!
 */