Session variables seem not to be saved

Putnik

Quite simple code:

<?
session_start();
$_SESSION['t'.time()]     = "ok";
echo "<pre>".print_r($_SESSION, 1)."</pre>";
?>

shows, as expected, something like

Array
(
    [t1330966834] => ok
    [t1330966835] => ok
    [t1330966836] => ok
)

after 3page reloads.

Let’s change a few symbols:

$_SESSION[time()]     = "ok";

(now without ‘t’) and I expect after few reloads something like

Array
(
    [t1330966834] => ok
    [t1330966835] => ok
    [t1330966836] => ok
    [1330967020] => ok
    [1330967021] => ok
    [1330967022] => ok
    [1330967023] => ok
)

But actually the result is absolutely different:

   Array
    (
        [t1330966834] => ok
        [t1330966835] => ok
        [t1330966836] => ok
        [1330967020] => ok
    )

We have 3 previous array cells ad one and only one ‘time’ cell – no matter how many times you reload the page. The time is correct, it different each second but only one cell without ‘t’!
Also I tried

$t =time();
$_SESSION[$t]     = "ok";

and even

$t =intval(time());
$_SESSION[$t]     = "ok";

But it’s remains only one cell with time.

Tested at php 5.2.13 and 5.3.10 at 2 different servers.
What am I doing wrong?