Undefined variable: text_cart_heading file cart.tpl in opencart
if you found this problem after installing theme, this is what you need to do :

1. open controlpanel and look for the : yourwebsite.com/catalog/controller/common/cart.php (my problem happen here)
2. add this following code : or depending on your error $data['text_cart_heading'] = $this->language->get('text_cart_heading');

this error happen because you do not call the function in cart.php but you write it on your template in cart.tpl

Thank you