I know `Notice: Undefined index` error is about lack of `isset()`.

Below statements doesn't get any errors or notices :

```
if (isset($_SESSION['userid'])) {
    $userid = $_SESSION['userid'];  
}

```

But following statements, get `Notice: Undefined index` :

```
$userid = $_SESSION['userid'] ? isset($_SESSION['userid']) : NULL;

```

Please tell me why when using shorthand `if/else` it get notice?