(PHP 5 >= 5.3.0, PHP 7) Checks whether the queue is empty.

(PHP 5 >= 5.3.0, PHP 7) Return current node pointed by the iterator

(PHP 5 >= 5.3.0, PHP 7) Inserts an element in the queue by sifting it up.

(PHP 5 >= 5.3.0, PHP 7) Move to the next node