<?php
  class Cart {
    var $items;
    function add_item($artnr, $num) {
        $this->items[$artnr] += $num;
    }  
  } 
?>

<?php
  class NamedCart extends Cart {
    var $owner;
    function NamedCart($name) {
        $this->owner = $name;
    }
  } 
?>