Description:
------------
When using $_SESSION and trying to store there something in key=0, the whole session data is not 'magically' :) present in the next page load

Reproduce code:
---------------
<?php
session_start();
var_export($_SESSION);
$_SESSION[0]=array( ... );
?>

Expected result:
----------------
-- first load:
array()

-- second load
array( 0 => array( ... ) )

Actual result:
--------------
-- first load:
array()

-- second load
array()