PHP Stack

<?php
 
/*
 * @class: Stack
 * @author: Michael Scribellito
 * @date: 7-5-11
 */
 
require 'Node.php';
 
/*
 * Stack
 * =====
 * 
 * Represents a last-in-first-out (LIFO) stack of generic items. It supports
 * the usual push and pop operations, along with methods for peeking at the top
 * item, testing if the stack is empty, and iterating through the items in LIFO
 * order.
 * 
 * All stack operations are constant time.
 */
 
class Stack {
 
    // size of the stack
    private $N;
    // top of stack
    private $first;
 
    // create an empty stack.
    public function __construct() {
 
        $this->N = 0;
        $this->first = NULL;
 
    }
 
    // is the stack empty?
    public function isEmpty() {
 
        return $this->first == NULL;
 
    }
 
    // return the number of items in the stack.
    public function size() {
 
        return $this->N;
 
    }
 
    // add the item to the stack.
    public function push($item) {
 
        // save a link to the list
        $oldfirst = $this->first;
        // create a new node for the beginning
        $this->first = new Node();
        // set the instance variables in the new node
        $this->first->item = $item;
        $this->first->next = $oldfirst;
        // increment size
        $this->N++;
 
    }
 
    // return the item most recently added to the stack.
    // * throw an exception if the queue is empty.
    public function peek() {
 
        if ($this->isEmpty())
            throw new Exception("Stack underflow");
        return $this->first->item;
 
    }
 
    // delete and return the item most recently added to the stack.
    // * throw an exception if the queue is empty.
    public function pop() {
 
        if ($this->isEmpty())
            throw new Exception("Stack underflow");
        // save the most recently added item
        $item = $this->first->item;
        // remove the first node
        $this->first = $this->first->next;
        // decrement size
        $this->N--;
        return $item;
 
    }
 
}