PHP如何通过带尾指针的链表实现'队列'

<?php
require 'QueueByLinkedList.php';
$queue = new QueueByLinkedList();
$queue->enqueue("rr"); //入队
$queue->enqueue("tt"); //入队
$queue->enqueue("yy"); //入队
$queue->enqueue("uu"); //入队
$queue->enqueue("ii"); //入队
$queue->enqueue("oo"); //入队
echo $queue->toString(); //打印 rr->tt->yy->uu->ii->oo->null
echo "<br>";
echo $queue->dequeue(); //出队 打印 rr
echo "<br>";
echo $queue->dequeue(); //出队 打印 tt
echo "<br>";
echo $queue->dequeue(); //出队 打印 yy
echo "<br>";
echo $queue->toString(); //打印 uu->ii->oo->null
echo "<br>";
$queue->enqueue("11"); //入队
$queue->enqueue("22"); //入队
$queue->enqueue("33"); //入队
$queue->enqueue("44"); //入队
$queue->enqueue("55"); //入队
$queue->enqueue("66"); //入队
echo "<br>";
echo $queue->toString(); //打印 uu->ii->oo->11->22->33->44->55->66->null

<?php
require 'Queue.php';
/**
 * 带有尾指针的链表
 * Class LinkedListTail
 */
class QueueByLinkedList implements Queue
{
  private $head; //链表头部
  private $tail; //链表尾部
  private $size; //链表大小
  /**
   * 构造函数 初始化链表
   * QueueByLinkedList constructor.
   */
  public function __construct() {
    $this->head = null;
    $this->tail = null;
    $this->size = 0;
  }
  /**
   * 入队操作
   * @param $e
   */
  public function enqueue($e): void {
    if ($this->tail == null) {
      $this->tail = $this->head = new Node($e, null);
    } else {
      $node = new Node($e, null);
      $this->tail->next = $node;
      $this->tail = $node;
    }
    $this->size++;
  }
  /**
   * 出队操作
   * @return mixed
   */
  public function dequeue() {
    if ($this->size == 0) {
      return "队列已经是空的";
    }
    $node = $this->head;
    $this->head = $node->next;
    $this->size--;
    if ($node->next == null) {
      $this->tail = null;
    }
    return $node->e;
  }
  public function getFront() {
    if ($this->size == 0) {
      return "队列已经是空的";
    }
    return $this->head->e;
  }
  public function getSize() {
    return $this->size;
  }
  /**
   * 判断队列是否为空
   * @return bool
   */
  public function isEmpty(): bool {
    return $this->size == 0;
  }
  public function toString() {
    $str = "";
    for ($node = $this->head; $node != null; $node = $node->next) {
      $str .= $node->e . "->";
    }
    $str .= "null";
    return $str;
  }
}
class Node
{
  public $e;//节点元素
  public $next; //下个节点信息
  /**
   * 构造函数 设置节点信息
   * Node constructor.
   * @param $e
   * @param $next
   */
  public function __construct($e, $next) {
    $this->e = $e;
    $this->next = $next;
  }
}

<?php
interface Queue
{
  public function enqueue($e): void;//入队
  public function dequeue();//出队
  public function getFront();//获取前端元素
  public function getSize();//获取队列大小
  public function isEmpty();//判断队列是否为空
}