[Typo3] hot to check if user is logged on

[Justin Kadima]

Hello to all,

Do you have any ideea how to check from inside PHP code(USER_INT) if a
user is logged? .Is there any variable that is set to TRUE or someting
like that, when a frontend user is logged?
I'm making a frontend extension in which I have different output based
on the user status.

Thanks, in advance