A simple PHP implementation of a Stack

Stack.php

<?php

// I could use array_push and array_pop with an array, as PHP already implements internally a stack with a doubly linked list


class Node {

    public $data;
    public $next;

    public function __construct($data)
    {
        $this->data = $data;
        $this->next = NULL;
    }

    public function getData()
    {
        return $this->data;
    }

}


class Stack {

    private $top;

    public function __construct()
    {
        $this->top = NULL;
    }

    public function peek()
    {
        if($this->isEmpty()) throw new Exception("The stack is empty");
        return $this->top->getData();
    }

    public function isEmpty() {
        return $this->top == NULL;
    }

    public function push($data) {

        $new_node = new Node($data);
        $new_node->next = $this->top;
        $this->top = &$new_node;

    }

    public function pop() {
        if($this->isEmpty()) throw new Exception("The stack is empty");
        $top = $this->top;
        $this->top = $top->next;
        return $top->getData();
    }

}

$s = new Stack();
$s->push(3);
$s->push(4);
echo $s->peek()."\n";
$s->pop();
echo $s->peek()."\n";