<?php

class Node 
{    
    // always initialize properties
    public mixed $data = null;
    public ?Node $next = null;
    
    public function __construct(mixed $data) 
    {
        $this->data = $data;        
    }
}

class Stack 
{    
    // always initialize properties
    public ?Node $head = null; 
    
    public function push(mixed $data) : void 
    {
        $node = new Node($data);          
        $node->next = $this->head;
        $this->head = $node;
    }
    public function pop() : mixed 
    {
        if (!$this->head) 
        {
            return 'Stack empty';
        }
        $data = $this->head->data;
        $this->head = $this->head->next;    
        
        return $data;
    }
}

$ll = new Stack();
echo 'pop(): ' . $ll->pop() . PHP_EOL;
for ($i = 1; $i < 4; $i++) { $ll->push($i); }
var_dump($ll);
echo 'pop(): ' . $ll->pop() . PHP_EOL;
echo 'pop(): ' . $ll->pop() . PHP_EOL;
echo 'pop(): ' . $ll->pop() . PHP_EOL;
echo 'pop(): ' . $ll->pop() . PHP_EOL;
var_dump($ll);