Description:
------------
Attempts to use a DateTime object that has been serialized/unserialized produces an error.

Reproduce code:
---------------
<?php
	$dttNow = new DateTime('now');
	print $dttNow->format(DateTime::ISO8601) . '<br/>';
	$strSerialized = serialize($dttNow);
	
	$dttRestored = unserialize($strSerialized);
	print $dttRestored->format(DateTime::ISO8601) . '<br/>';
?>

Expected result:
----------------
2006-12-13T12:26:39-0800
2006-12-13T12:26:39-0800

Actual result:
--------------
2006-12-13T12:26:39-0800

Warning: DateTime::format() [function.DateTime-format]: The DateTime object has not been correctly initialized by its constructor in /home/wcc/wwwroot/test.php on line 7