(PHP 5 >= 5.3.0, PHP 7) Constructs a new queue implemented using a doubly linked list

(PHP 5 >= 5.3.0, PHP 7) Adds an element to the queue.

(PHP 5 >= 5.3.0, PHP 7) Dequeues a node from the queue

(PHP 5 >= 5.3.0, PHP 7) Sets the mode of iteration