title: OpenCart Call Different Controller
body: <p>I have a custom module, and now want to call the <code>add()</code> function from <code>checkout/cart</code>. How do I call the controller and function?</p>
  
  <p>I have tried <code>$this-&gt;load-&gt;controller('checkout/cart');</code> but this returns a fatal exception.</p>
  
  <p>I am using OpenCart v 1.5.6.4</p>