function polska($str) {
    $stack = new SplStack();
    foreach(explode(' ', $str) as $item) {
        if(in_array($item, array('+', '-', '*', '/'))) {
            if($stack->count() < 2 ) {
                throw new Exception("invalid count of arguments");
            }
            $arg_1 = $stack->pop();
            $arg_2 = $stack->pop();
            switch($item) {
                case '+':
                    $res = $arg_1 + $arg_2;
                    break;
                case '-':
                    $res = $arg_1 - $arg_2;
                    break;
                case '*':
                    $res = $arg_1 * $arg_2;
                    break;
                case '/':
                    $res = $arg_1 / $arg_2;
                    break;
            }
            $stack->push($res);
        } else {
            if(!is_numeric($item)) {
                throw new Exception("invalid value {$item}");
            }
            $stack->push($item);
        }
    }
    return $stack->pop();
}

echo polska('5 3 8 + *') . "\n";
echo polska('5 3 + 8 *') . "\n";
echo polska('5 *') . "\n";