One of the codility task implemented by me (Word machine with capability add, duplicate, pop, sum and subtract from the stack)

word-machine.php

<?php

function solution($S)
{
    $stack = [];

    foreach(parseCommand($S) as $command) {
        try {
            $stack = performOperation($command)($stack);
        } catch (Exception $e) {
            return -1;
        }
    }

    return $stack;
}

function parseCommand($string)
{
    return explode(' ', $string);
}

function performOperation($command)
{
    return function($stack) use ($command) {
        $newStack = $stack;

        switch($command) {
            case is_numeric($command):
                array_push($newStack, $command);
                break;

            case "DUP":
                array_push($newStack, end($stack));
                break;

            case "POP":
                array_pop($newStack);
                break;

            case "+":
                $lastElement = array_pop($newStack);
                $preLastElement = array_pop($newStack);
                array_push($newStack, $lastElement + $preLastElement);
                break;

            case "-":
                $lastElement = array_pop($newStack);
                $preLastElement = array_pop($newStack);

                if ($preLastElement === null || ($lastElement - $preLastElement) < 0) {
                    throw new Exception('Not enough items in a stack');
                }

                array_push($newStack, $lastElement - $preLastElement);
                break;
        }

        return $newStack;
    };
}

print_r(solution('3 3 2 DUP POP + -')); // result is 2