!isset, is_null, empty: The Good, the Bad and the Ugly

Hi everyone, you programmers trying to grasp the intricacies of PHP. As coders, our raw material is, and will be, information and knowledge. But lack of knowledge does not give us some information, too? Is of this absence that we will discuss today: How to know if we don't know something in PHP. And our main tools are isset, is_null and empty. Want to know more about them? Which one is The Good, The Bad and The Ugly?