<?php

class Node 
{
    public ?int $data = null;   
    public ?Node $next = null;

    public function __toString() : string 
    {
        return "$this->data $this->next";
    }
}

class Stack 
{
    private ?Node $head = null;
    
    public function push(int $data) : void 
    {
        $node = new Node;
        $node->data = $data;
        $node->next = $this->head;
        $this->head = $node;
    }

    public function pop() : void 
    {
        if (!$this->head) { return; }
        echo $this->head->data . " popped from stack\n";
        $this->head = $this->head->next;
        echo $this;
    }

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

$stk = new Stack;
for ($i=1; $i<4; $i++) { $stk->push($i*10); }
print($stk);
$stk->pop();