Comment by dmiladinov

In order to use variables from the scope that the anonymous function came from, you have to explicitly import them into the anonymous function with the use[1] block:

    public function getTotal($tax)
    {
        $total = 0.00;
        
        $callback =
            function ($quantity, $product) use ($tax, &$total)
            {
                $pricePerItem = constant(__CLASS__ . "::PRICE_" .
                    strtoupper($product));
                $total += ($pricePerItem * $quantity) * ($tax + 1.0);
            };
        
        array_walk($this->products, $callback);
        return round($total, 2);
    }

In other languages that have true closures, the anonymous function "closes over" the lexical scope in which it was declared.

[1]: http://www.php.net/manual/en/functions.anonymous.php