From

(($mode & \SplDoublyLinkedList::IT_MODE_LIFO) ? 'IT_MODE_LIFO' : 'IT_MODE_FIFO').' | '.(($mode & \SplDoublyLinkedList::IT_MODE_KEEP) ? 'IT_MODE_KEEP' : 'IT_MODE_DELETE'

Problem is that \SplDoublyLinkedList::IT_MODE_KEEP == 0 (AFAIK on both php and hhvm). So ($mode & IT_MODE_KEEP) == ($mode & 0) == 0, i.e. always falsy, and IT_MODE_KEEP is never printed, even for default-constructed \SplDoublyLinkedLists, where mode == IT_MODE_FIFO | IT_MODE_KEEP (==0), per php doc.