If $mySession is not set

PHP
1 
<?php session_start(); 

if (!isset($_session['mySession'])) {
    $_session['mySession'] = 'mySess';}
echo $_session['mySession']; 

$_session['mySession']='mySessSet';
?>

I have the code above at http://dot.kr/x-test/IFsession1.php .

IF mySession is not set(When I open the file), the value of mySession will be ‘mySess’ (It will print “mySess” on my browser).

IF mySession is set (When I refresh the file), the value of mySession will be “mySessSet” because mySession is already set (It should print “mySessSet”).

But The result is not what I expected.

Although I reflesh the page, it prints just “mySess” instead of “mySessSet”.

What’s wrong in my code?
(I want that it prints “mySess” when I open the file for the first time and it prints “mySessSet” when I open the file for the second time.)

2

change $_session to $_SESSION