In PHP, what is a closure and why does it use the "use" identifier?

Question

I'm checking out some PHP 5.3.0 features and ran across some code on the site that looks quite funny:

public function getTotal($tax)
{
    $total = 0.00;

    $callback =
        /* This line here: */
        function ($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);
}

as one of the examples on anonymous functions.

Does anybody know about this? Any documentation? And it looks evil, should it ever be used?

Answer 1

A simpler answer.

function ($quantity) use ($tax, &$total) { .. };

1. The closure is a function assigned to a variable, so you can pass it around
2. A closure is a separate namespace, normally, you can not access variables defined outside of this namespace. There comes the use keyword:
3. use allows you to access (use) the succeeding variables inside the closure.
4. use is early binding. That means the variable values are COPIED upon DEFINING the closure. So modifying $tax inside the closure has no external effect, unless it is a pointer, like an object is.
5. You can pass in variables as pointers like in case of &$total. This way, modifying the value of $total DOES HAVE an external effect, the original variable's value changes.
6. Variables defined inside the closure are not accessible from outside the closure either.
7. Closures and functions have the same speed. Yes, you can use them all over your scripts.

As @Mytskine pointed out probably the best in-depth explanation is the RFC for closures. (Upvote him for this.)