function calculate_price($cart)
{
  // sum total price for all items in shopping cart
  $price = 0.0;
  if(is_array($cart))
  {
    $conn = db_connect();
    foreach($cart as $isbn => $qty)
    {  
      $query = "select price from books where isbn='$isbn'";
      $result = mysql_query($query);
      if ($result)
      {
        $item_price = mysql_result($result, 0, 'price');
        $price +=$item_price*$qty;
      }
    }
  }
  return $price;
}