Can't unset string offsets

[Phalanxer]

I really don't quite understand this problem:

 

Cannot unset string offsets

 

if (isset($_SESSION['tablea']['numbers1-6'])) {
$_SESSION['tablea']['numbers1-6prior'] = $_SESSION['tablea']['numbers1-6'];
unset($_SESSION['tablea']['numbers1-6']);
}

 

The problem is with the last line - unset($_SESSION['tablea']['numbers1-6']);

 

Can someone please tell me what the problem is? I don't understand.

[memfiss]

in this part no problems

$_SESSION['tablea']['numbers1-6'] = 1;
if (isset($_SESSION['tablea']['numbers1-6'])) 
{
$_SESSION['tablea']['numbers1-6prior'] = $_SESSION['tablea']['numbers1-6'];
unset($_SESSION['tablea']['numbers1-6']);
}
var_dump($_SESSION['tablea']['numbers1-6']);

Notice: Undefined index: numbers1-6 in /var/www/absolux/test/run.php on line 8

NULL