fix reported require_once() conflict with plugins

...We have a problem using the (external) module "game" with it. The module game is needed for some courses in primary-schools and has some nice games like millionaire, hangman, cryptex, cross ...

But if we are using the Zebra-theme - the games doesn't start => error site not found (or something else). With all built-in themes the games works fine. If I turn the debug-mode on, the error_log from Apache had the following entries:

PHP Notice:  Undefined variable: DB in /MOODLE/www/htdocs/MASTER1/mod/game/header.php on line 15

PHP Fatal error:  Call to a member function get_record() on a non-object in /MOODLE/www/htdocs/MASTER1/mod/game/header.php on line 15

Why the DB wasn't found (I thought it is a session-variable)? Have you any idea? Did the theme override the default header or something else?