Wow, haven't seen that register page in a while...
Did you get these errors before or after submitting the form? These errors will show when attempting to set a $_POST variable when a POST request was not made (form submission).
To prevent them, you'd have to adjust the code to not reference any $_POST variables outside of an actual POST request. However those warnings alone shouldn't prevent the form from working, as the $_POST variables would exist after someone submits the form to register an account.
Hope this makes sense. For more details, check PHP documentation for the $_POST variable.