<p>Typically your ability to take extra units after hitting your minimum required number as a PhD student hinges on the permission of your advisor. Some are happy to have their students expand their horizons and learn about everything they’re interested in (I had a friend that took a class every term even after he was “done” with classes.) while others won’t want you to do anything outside of lab. This is definitely something to consider when choosing a lab to do your PhD in. If you really are interested in taking those classes you’ll want an advisor that supports you.</p>