Question

This has got me very confused and I can’t seem to find an answer to this question. A clear and simple clarification would be nice.

Answer 1

A closure is a function that is evaluated in its own environment, which has one or more bound variables that can be accessed when the function is called. They come from the functional programming world, where there are a number of concepts in play. Closures are like lambda functions, but smarter in the sense that they have the ability to interact with variables from the outside environment of where the closure is defined.

The use() keyword let’s you import variables from outside the function environment, inside the function. Variables to be imported from the outside environment are specified in the use clause of the closure function definition. By default, they are passed by value. So let’s say the function has no parameters, but you wan’t it to use a variable you already have.

$string = "Hello World!";
$closure = function() use ($string) { echo $string; };

This is useful when you need to create a function what must be used as callback somewhere else, and can only have defined parameters. The use() keyword let’s you use other variables in addition to the ones you pass as function arguements. For example on the php.net example: http://php.net/manual/en/functions.anonymous.php

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);
    }

$callback must only have two parameters, because array_walk will only allow that much:

Typically, funcname takes on two parameters. The array parameter’s
value being the first, and the key/index second.

So what can we do? We call use() to add other variables that are not the $callback’s scope, but in scope of the environment it is being called in.