PHP's static keyword

I quite like PHP's static keyword (C has the same thing). It lets you create a persistent variable that is local to a function, without having to add an instance variable. Useful for lazy loading:

    private function getInvitation() {
        static $invitation = null;
        if (! $invitation) {
            ... create invitation ...
            $invitation = ...;            
        }
        return $invitation;
    }