Magento add to cart from url, useful for email promos or add to cart from offsite links

magento-add-to-cart-from-query-string.php

<?php
/**
 * add to cart from url, useful for email promos
 * based on http://www.magentocommerce.com/wiki/4_-_themes_and_template_customization/catalog/adding_a_product_to_the_cart_via_querystring
 * which no longer works because of session validation of form_key
 * usage: link to add.php?product=[id]&qty=[qty]
 */

require 'app/Mage.php';
Mage::app();
Mage::getSingleton('core/session', array('name' => 'frontend'));

// init params
$productId = isset($_GET['product']) ? (int) $_GET['product'] : null;
$qty = isset($_GET['qty']) ? (int) $_GET['qty'] : 1;

// if invalid params, send to homepage
if ( !$productId || !$qty ) {
  header('Location: ' . Mage::getUrl(''));
  exit;
}

// load product and get url
$_product = Mage::getModel('catalog/product')->load($productId);
$url = Mage::helper('checkout/cart')->getAddUrl($_product, ['qty'=>$qty]);
header('Location: ' . $url);