Difference between `if (isset($_SESSION))` and `if ($_SESSION)`?

Question

I\'ve noticed that frequently people simply write

while I have been using:

Answer 1

In PHP, if a variable does not exist (is "unset") then PHP will spit out an E_NOTICE error, create the missing variable, and assign it to NULL.

If you do not want your scripts filled with annoying error messages that leak information about your server and script - then use isset() to test before trying to access a value.

Basically, you're asking PHP to get username out of $_SESSION and PHP is like, "there is no username"!