<?php
class element{
    var $val;
    var $next;
}

class stack{
    var $current = null;

    function push($val){
        $element = new element();
        $element->val = $val;
        $element->next = $this->current;
        $this->current = $element;
    }

    function pop(){
        if($this->current == null) return -1;
        $tmp = $this->current;
        $this->current = $tmp->next;
        return $tmp->val . "\n";
    }
}

$stack = new stack();

$stack->push(5);
$stack->push(4);
$stack->push(2);

echo $stack->pop();
echo $stack->pop();
echo $stack->pop();
echo $stack->pop();