Restrict Products in Cart Based on Other Products in Cart

[lat9]

Put the following code into /includes/extra_cart_actions/one_per_cart.php; you'll need to edit the ONE_PER_CART_PRODUCT_IDS value to reflect the product_ids of your restricted items:

<?php
/**
 * Extra shopping Cart actions supported.
 *
 * The main cart actions supported by the current shoppingCart class.
 *
 * NOTE: Many of the defines used in this module have been included by
 * class.init_quote.php's inclusion of /includes/quotes_status_array.php.
 *
 */
define('ONE_PER_CART_PRODUCT_IDS', '14,4,6,11,15'); /*Identify the product_ids of the products that must be ordered separately*/
define('ERROR_ONE_PER_CART', 'Sorry, but you can\'t add "%s" to an order that already contains another item.');
define('ERROR_ONE_PER_CART_COMBINE', 'Sorry, but you can\'t add another item to an order that already contains "%s".');

switch ($_GET['action']) {
  case 'add_product': 
    $onePerCartIDs = explode (',', ONE_PER_CART_PRODUCT_IDS);
    $inCartName = '';
    foreach ($onePerCartIDs as $theID) {
      if ($theID == $_POST['products_id']) {
        if ($_SESSION['cart']->count_contents() > 0) {
          $error = true;
          $messageStack->add('product_info', sprintf(ERROR_ONE_PER_CART, zen_get_products_name((int)$_POST['products_id'])));
        }
      } elseif ($_SESSION['cart']->in_cart_mixed($theID) != 0) {
        $inCartName = zen_get_products_name($theID);
      }
    }
    if (!$error && $inCartName != '') {
      $error = true;
      $messageStack->add('product_info', sprintf(ERROR_ONE_PER_CART_COMBINE, $inCartName));
    }
  }

  if ($error) {
    unset($_GET['action']);
  }
  break;
}