Description:
------------
Particular combination of session data causes "PHP Warning:  session_start(): Failed to decode session object."
Found in our web application but recreated in the test CLI script below.

Steps to reproduce:
1) run this file once. Session data is saved.
2) Run again. Session cannot be decoded.
3) Remove session file /tmp/sess_sessiontest. If it wasn't already removed by the session_decode failing.
4) Comment out the $_SESSION['boogie'] line
5) Run again, and again and again and again.......No bug!



Test script:
---------------
<?php

$session_id = 'sessiontest';

session_id($session_id);
session_start();
$_SESSION['boogie'] = 1;//NOTE: Comment out this line for step 4

$_SESSION['obj1'] = new stdClass();
for ( $x=2; $x < 20; $x++) {//NOTE: In my tests the number needed varied but was always between 10 and 20
     cyclic_ref($x);
}


function cyclic_ref($num) {
         $_SESSION['obj'.$num] = new stdClass();
         $_SESSION['obj'.$num]->test = new stdClass();//NOTE: No bug if try commenting out this too.
         $_SESSION['obj'.$num]->obj1 = $_SESSION['obj1'];
}


Expected result:
----------------
When session successfully decoded no output.

Actual result:
--------------
#first run - success. Session written.
$ php sessiontest.php

#second run. Fail. Session cannot be decoded.

~$ php sessiontest.php
PHP Warning:  session_start(): Failed to decode session object. Session has been destroyed in /path/sessiontest.php on line 16