Description:
------------
The following session is stored in my temp directory:

old_var2|s:4:"foo2";

I try to iterate over the $_SESSION var (using foreach) to unset some session keys, in particular those who start with "old_" (see the attached code). But I can't.

Besides, if after this iteration I try to modify the $_SESSION var nothing happens.

I'm using IIS 5.1 (blame my boss).

Reproduce code:
---------------
session_start();

$prefix = 'old_';
	
foreach ($_SESSION as $key => $value) {
	if (substr($key, 0, 4) == $prefix) {
		unset($_SESSION[$key]);
	}
}
	
$_SESSION['var1'] = 'foo';

Expected result:
----------------
This change in the session file:

var1|s:3:"foo";

Actual result:
--------------
old_var2|s:4:"foo2";