Brainstorming the interfaces for Deque, Queue, and Stack.

Deque.php

<?php

namespace Spl;

/**
 * Indicates that a structure is designed to operate efficiently at both the
 * front and the back of a dataset.
 * @template T
 */
interface Deque
    extends \Countable, \IteratorAggregate
{
    /** 
     * Prepends the given value to the front of the structure.
     * @psalm-param T $value
     */
    function push_front($value);

    /** 
     * Removes and returns the first value of the structure.
     *
     * @psalm-return T The value that was removed from the front of the structure.
     *
     * @throws EmptyContainerException if the structure is empty.
     */
    function pop_front();

    /** 
     * Returns the first value of the structure without removing it.
     * @psalm-return T The value at the front of the structure.
     * @throws EmptyContainerException if the structure is empty.
     */
    function front();

    /** 
     * Appends the given value to the back of the structure.
     * @psalm-param T $value
     */
    function push_back($value);

    /** 
     * Removes and returns the last value of the structure, the back of the structure.
     *
     * @psalm-return T The value that was removed from the back of the structure.
     *
     * @throws EmptyContainerException if the structure is empty.
     */
    function pop_back();

    /** 
     * Returns the last value of the structure without removing it.
     * @psalm-return T The value at the back of the structure.
     * @throws EmptyContainerException if the structure is empty.
     */
    function back();
}

Queue.php

<?php

namespace Spl;

/**
 * A structure with First-In, Last-Out (FIFO) behavior.
 * @template T
 */
interface Queue
    extends \Countable, \IteratorAggregate
{
    /** 
     * Appends the given value to the back of the structure.
     * @psalm-param T $value
     */
    function push_back($value);

    /** 
     * Removes and returns the first value of the structure.
     *
     * @psalm-return T The value that was removed from the front of the structure.
     *
     * @throws EmptyContainerException if the structure is empty.
     */
    function pop_front();

    /** 
     * Returns the first value of the structure without removing it.
     * @psalm-return T The value at the front of the structure.
     * @throws EmptyContainerException if the structure is empty.
     */
    function front();

}

Stack.php

<?php

namespace Spl;

/**
 * A structure with First-In, Last-Out (FIFO) behavior.
 * @template T
 */
interface Stack
    extends \Countable, \IteratorAggregate
{
    /** 
     * Appends the given value to the back of the structure.
     * @psalm-param T $value
     */
    function push_back($value);

    /** 
     * Removes and returns the last value of the structure, the back of the structure.
     *
     * @psalm-return T The value that was removed from the back of the structure.
     *
     * @throws EmptyContainerException if the structure is empty.
     */
    function pop_back();

    /** 
     * Returns the last value of the structure without removing it.
     * @psalm-return T The value at the back of the structure.
     * @throws EmptyContainerException if the structure is empty.
     */
    function back();

}