<?php

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

    public function __toString() {        
        $output = ($this->next !== null) ? "$this->next, " : "";
        return $output . $this->data;
    }
}

class Stack {
    private $top;

    public function __construct() {
        echo $this;
    }

    public function push($data) {
        $node = new Node($data);
        $node->next = $this->top;
        $this->top = $node;
        echo "push $data\n";
        echo $this;
    }

    public function pop() {
        if ($this->top === null) {
            return null;
        }
        $data = $this->top->data;
        $this->top = $this->top->next;
        echo "pop $data\n";
        echo $this;
        return $data;
    }

    public function __toString() {
        return "Stack: [$this->top]\n";
    }
}

$stack = new Stack();

for ($i = 1; $i < 4; $i++) { 
    $stack->push($i); 
}

for ($i = 1; $i < 6; $i++) { 
    $stack->pop(); 
}