Stack - Data Structure using PHP

Stack.php

<?php
/**
  * Implementation of Stack using PHP
  * @author Al- Imran Ahmed
  */
class Node{
  public $value;
  public $next;
}

class Stack{
  private $element;
  private $top;
  
  /**
    * check whether the stack is empty or not
    * @return boolean
    * public function isEmpty(){ return false; }  ;stub
    */
  public function isEmpty(){
    return $this->top == null;
  }
  
  /**
    * Remove an element form the stack
    * @return $element
    * public function pop(){ return 0; } ;stub
    */
  public function pop(){
    if(!$this->isEmpty()){
      $value = $this->top->value;
      $this->top = $this->top->next; 
      return $value;
    }
    return null;
  }

  /**
    * Insert an element on the stack
    * @param $element
    * public function push($value){
    */ 
  public function push($value){
    $oldtop = $this->top;
    $this->top = new Node();
    $this->top->value = $value;
    $this->top->next = $oldtop;
  }
}

/*
//Client code
$stack = new Stack();
$stack->push(1);
$stack->push(2);
$stack->push(333);
$stack->push("end");

while(!$stack->isEmpty()){
  echo $stack->pop()."\n";
}*/