Magento Add to cart formkey

Method 1:

echo $this->getBlockHtml('formkey'); // show a form hidden input field.

echo htmlspecialchars($this->getBlockHtml('formkey'));

Method 2:

$_formKey = Mage::getSingleton('core/session')->getFormKey();

echo '<input name="form_key" type="hidden" value="' . $_formKey . '">';