Description:
------------
I am french, sorry for the language.

This code don't add key when i refresh this all seconds:
<?php
session_start();
$_SESSION[time()]    = time();

print_r($_SESSION);
?>

But this code is good:
<?php
session_start();
$_SESSION['a'.time()]    = time();

print_r($_SESSION);
?>

Reproduce code:
---------------
<?php
session_start();
$_SESSION[time()]    = time();

print_r($_SESSION);
?>

Expected result:
----------------
<?php
session_start();
$_SESSION['a'.time()]    = time();

print_r($_SESSION);
?>

Actual result:
--------------
Array ( [1164901301] => 1164901301 )