Zend_Session_Namespace allows invalid namespace names like "0" to be used as the namespace. This is a problem because 0 is not a valid key of $_SESSION. From the PHP manual: http://php.net/session "The keys in the $_SESSION associative array are subject to the same limitations as regular variable names in PHP, i.e. they cannot start with a number and must start with a letter or underscore. For more details see the section on variables in this manual." This is because of register_globals compatibility. Also, see bug #42472: http://bugs.php.net/bug.php?id=42472 Trying to set $_SESSION[0] produces an E_NOTICE message of the following form: Notice: Unknown: Skipping numeric key 0. in Unknown on line 0 $_SESSION[0] is never stored persistently either. Zend_Session_Namespace should validate the namespace passed so that it meets the requirements specified in the PHP manual: It must start with a letter or underscore.