PHP Interview Question?

1. How I will check that user is logged in or not?
2. I want to make it a function and I want to use it in each page and after login I want to go in current page?
3. How do we know browser properties?
4. If you have to work with dates in the following format: “Tuesday, February 14, 2006 @ 10:39 am”, how can you convert them to another format, that is easier to use?
5. What is difference between require_once(), require(), include()?