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

Question

I\'ve noticed that frequently people simply write

while I have been using:

Answer 1

say you set a variable = to false...

$variable = false;

This will not return anything, because it is making sure that the variable is not null, false or empty('')...

if($variable){
    echo'something';
}

This will work regardless of what we set the variable to, as long as we set it... It can be false, str, int, anything except null!

if(isset($variable)){
    echo'something';
}