variable frustration

Forgive me asking such simple questions but I’m a bit confused.
On my index.php page I have this:

<?php
session_start();
$_SESSION[‘isadmin’] = ‘NO’;
?>

On a member login page I have this:

<?php
$flag = $_SESSION[‘isadmin’];
if ($flag == ‘YES’){
echo $flag;
}
?>

When I run it I get:

PHP Notice: Undefined variable: _SESSION in c:Inetpubwwwrootmemberlogin.php on line 67

Can someone explain why it is flagging _SESSION as an undefined variable?

Thanks,
Rut

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