gistfile1.php

<?php
/**
 * @author Evgeny Shpilevsky <evgeny@shpilevsky.com>
 */

class Purchase
{

    /**
     * @var int
     */
    protected $cost;

    /**
     * @param int $cost
     */
    public function __construct($cost)
    {
        $this->cost = $cost;
    }

    /**
     * @return int
     */
    public function getCost()
    {
        return $this->cost;
    }


}

class Cart implements IteratorAggregate
{

    /**
     * @var Purchase[]
     */
    protected $purchases;

    /**
     * @param Purchase $purchase
     */
    public function addPurchase(Purchase $purchase)
    {
        $this->purchases[] = $purchase;
    }

    /**
     * @return int
     */
    public function getCost()
    {
        $cost = 0;
        foreach ($this->purchases as $purchase) {
            $cost += $purchase->getCost();
        }
        return $cost;
    }

    /**
     * @return ArrayIterator|Traversable
     */
    public function getIterator()
    {
        return new ArrayIterator($this->purchases);
    }

}

$cart = new Cart();
$cart->addPurchase(new Purchase(10));
$cart->addPurchase(new Purchase(15));

var_dump($cart->getCost()); // 25

foreach($cart as $purchase) {
    var_dump($purchase->getCost());
}