install.xml

<modification>
  <name>GET Cart API</name>
  <version>1.0</version>
  <link>https://github.com/releu/get-cart-api</link>
  <author>releu</author>
  <code>get-cart-api</code>

  <file path="catalog/controller/product/product.php">
    <operation>
      <search ><![CDATA[public function index()]]></search>
      <add position="before"><![CDATA[public function addtocart() {

    $product_id = $this->request->get['product_id'];
    $quantity = (isset($this->request->get['quantity']) ? $this->request->get['quantity'] : 1);
    $option = array();
    $recurring_id = 0;

    // Check if product exist
    $this->load->model('catalog/product');

    $product_info = $this->model_catalog_product->getProduct($product_id);

    if ($product_info) {
      $product_options = $this->model_catalog_product->getProductOptions($product_id);

      foreach ($product_options as $product_option) {
        if ($product_option['required'] && empty($option[$product_option['product_option_id']])) {
          $this->response->redirect($this->url->link('product/product', 'product_id=' . $product_id));
          exit;
        }
      }

      $this->cart->add($product_id, $quantity, $option, $recurring_id);

      unset($this->session->data['shipping_method']);
      unset($this->session->data['shipping_methods']);
      unset($this->session->data['payment_method']);
      unset($this->session->data['payment_methods']);

      $this->response->redirect($this->url->link('checkout/cart'));

    } else {
      $this->response->redirect($this->url->link('common/home'));
    }
  }]]></add>
    </operation>
  </file>
</modification>