Online PHP editor | output for 0LpJ8

<?php // : counter: create 0 , does> dup @ tuck 1+ swap ! ; // counter: fred ok // fred // <1> 140599575159712 // After does> // <2> 140599575159712 140599575159712 // After dup // <2> 140599575159712 0 // After @ // <3> 0 140599575159712 0 // After tuck // <3> 0 140599575159712 1 // After 1+ // <3> 0 1 140599575159712 // After swap // <1> 0 // After ! global $__STACK; $__STACK = []; function dup() { error_log('dup'); global $__STACK; $top = reset($__STACK); array_push($__STACK, $top); } function at() { error_log('at'); global $__STACK; $top = array_pop($__STACK); array_push($__STACK, $top->cell); } function tuck() { error_log('tuck'); global $__STACK; $top = array_pop($__STACK); $second = array_pop($__STACK); array_push($__STACK, $top); array_push($__STACK, $second); array_push($__STACK, $top); } function swap() { error_log('swap'); global $__STACK; $top = array_pop($__STACK); $second = array_pop($__STACK); array_push($__STACK, $top); array_push($__STACK, $second); } function store() { error_log('store'); global $__STACK; $address = array_pop($__STACK); $value = array_pop($__STACK); $address->cell = $value; } function one_plus() { error_log('one_plus'); global $__STACK; $top = array_pop($__STACK); $top++; array_push($__STACK, $top); } class counter { public $cell = 0; public function __invoke() { global $__STACK; error_log('counter::__invoke'); array_push($__STACK, $this); dup(); at(); tuck(); one_plus(); swap(); store(); } } $fred = new counter(); $fred(); $fred(); $fred(); var_dump($__STACK);