Trouble with session serializer

I am finishing a PHP session serializer module and am seeing wierd things. I seems to work OK, but doesn't work with squirrelmail. It works mostly but there seems to be one type that isn't working and I can't seems to find it. If you want to help: download and install xmldbx in your PHP, copy xmldbx.php to some directory and see if it works, then try it with your code and see if there is something not working. Thanks.