undo.php

<?php
class db{
	static $count=0;//current data
	static $undos=array();//stack
}

function  add($n){
	db::$count+=$n;
	db::$undos[]=function() use($n){
		db::$count-=$n;
		echo "undo:$n\n";
	};
		echo  db::$count." current\n";
}

function undo(){
	if(db::$undos){
		$func=array_pop(db::$undos);
		$func();
		echo " ".db::$count."\n";
	};
}

add(1);add(2);add(3);/*undo();*/add(4);
undo();undo();undo();
/**
simple Command Pattern example which performs Undo operation on a calculator:
http://www.php.net/manual/pt_BR/language.oop5.patterns.php#106747

JavaScript Undo Manager:
https://github.com/ArthurClemens/Javascript-Undo-Manager

How to write a simple undo system for your app :
http://www.javascriptkata.com/2010/03/29/how-to-write-an-simple-undo-system-for-your-app/
*/
?>