php栈的数据结构

<?php
class Stack{
  const MAXSIZE = 4;// 栈最大容量
  private $top = -1;
  private $stack = array();// 利用数组存储数据
  public function __construct(){
    $this->stack = array();
  }
  // 入栈
  public function push($ele){
    if ($this->top >= self::MAXSIZE-1){
      echo 'stack is full...';
      return false;
    }
    $this->stack[++$this->top] = $ele;// 此处必须是++i，先计算再使用
  }
  // 出栈，返回出栈元素
  public function pop(){
    if ($this->top == -1){
      echo 'stack is empty...';
      return false;
    }
    $ele = $this->stack[$this->top];
    unset($this->stack[$this->top--]);// 此处必须是i--，先使用再计算（注意出栈和入栈的区别）
    return $ele;
  }
  // 遍历栈
  public function show(){
    if ($this->top == -1){
      echo 'stack is empty...';
      return false;
    }
    for($i=$this->top; $i>-1; $i--){
      echo $this->stack[$i].'<br/>';
    }
  }
}
$stack = new Stack;
$stack->push(1);
$stack->push(2);
$stack->push(3);
$stack->push(4);
//print_r($stack);
$stack->show();
$a = $stack->pop();
$a = $stack->pop();
$a = $stack->pop();
$stack->show();