What is the difference between $_GET and $_POST
This is a great question because an interviewer can tell how deeply you understand HTTP and the request/response. If you don't have good understanding of HTTP protocol, google around and get a grasp on it.
Good answer
Explain the HTTP protocol and how every request contains a method, generally(GET,POST,PUT,DELETE) and what each method signifies.
Bad answer
$_GET stores variables passed in url as query strings. $_POST stores variables past from using method = $_POST