Description:
------------
This is a very special case.

If an object of class that extends ArrayIterator is saved in session data and the class definition is not available in a later call, then the session data gets corrupted.

Call the example code three times to see the bug.

Reproduce code:
---------------
<?php
// start session
session_start();

// display session data
echo "<pre>".print_r($_SESSION,true)."</pre><br>";

// already initialized?
if (!isset($_SESSION['first']))
{
  // bug occures only if the class is not defined later
  // and only if it extends ArrayIterator
  class my_array extends ArrayIterator
  {
  }

  $_SESSION['first'] = "The first entry is ok";

  $t = new my_array();
  
  // at least one member needed
  $t[] = "this gets lost";

  $_SESSION['killer'] = $t;

  $_SESSION['this_is_going_to_be_messed_up'] = "The value of the entry is ok";

  $_SESSION['last'] = "The last entry is ok";
}


Expected result:
----------------
At first call:
Array
(
)

At second call:
Array
(
    [first] => The first entry is ok
    [killer] => __PHP_Incomplete_Class Object
        (
            [__PHP_Incomplete_Class_Name] => my_array
            [0] => this gets lost
        )

    [this_is_going_to_be_messed_up] => The value of the entry is ok
    [last] => The last entry is ok
)

At third call
Array
(
    [first] => The first entry is ok
    [killer] => __PHP_Incomplete_Class Object
        (
            [__PHP_Incomplete_Class_Name] => my_array
            [0] => this gets lost
        )

    [this_is_going_to_be_messed_up] => The value of the entry is ok
    [last] => The last entry is ok
)


Actual result:
--------------
At third call:

Notice: session_start() [function.session-start]: Unexpected end of serialized data in test.php on line 3

Array
(
    [first] => The first entry is ok
    [killer] => 
    [}this_is_going_to_be_messed_up] => The value of the entry is ok
    [last] => The last entry is ok
)