PHP: Cart Class with Anonymous function

Cart.php

<?php 
 
 /**
  * MIT License
  * ===========
  *
  * Copyright (c) 2012 MindCracker <[email protected]>
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files (the
  * "Software"), to deal in the Software without restriction, including
  * without limitation the rights to use, copy, modify, merge, publish,
  * distribute, sublicense, and/or sell copies of the Software, and to
  * permit persons to whom the Software is furnished to do so, subject to
  * the following conditions:
  *
  * The above copyright notice and this permission notice shall be included
  * in all copies or substantial portions of the Software.
  *
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
  * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
  * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
  * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
  * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
  * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  *
  * @category   PHP
  * @package    Cart Class
  * @subpackage none
  * @author     MindCracker <[email protected]>
  * @copyright  2012 MindCracker.
  * @license    http://www.opensource.org/licenses/mit-license.php  MIT License
  * @version    1.0
  * @link       http://smartweb.webz2feel.com
  */

 class Cart
 {
 	 const PRICE_BUTTER = 1.00;
 	 const PRICE_MILK   = 3.00;
 	 const PRICE_EGGS   = 6.85;

 	 public $_products = array();

 	 public function add($product,$quantity)
 	 {
 	 	$this->_products[$product] = $quantity;
 	 }

 	 public function getQuanitity($product)
 	 {
 	 	return isset($this->_products[$product]) ? $this->_products[$product] : false;
 	 }

 	 public function getTotal($tax)
 	 {
 	 	$total = 0.00;
 	 	$calback = function ($quantity,$product) use($tax, &$total)
 	 	{
 	 		$pricePetItem = constant(__CLASS__."::PRICE_".strtoupper($product));
 	 		$total += ($pricePetItem * $quantity) * ($tax + 1.0);
 	 	};

 	 	array_walk($this->_products, $calback);
 	 	return round($total,2);
 	 }
 }
 $my_cart = new Cart;
 $my_cart->add('butter',1);
 $my_cart->add('milk',3);
 $my_cart->add('eggs',6);
 echo $my_cart->getTotal(0.05);