Question

I had no idea something like this (javascript-esque inline function) would work in PHP. As someone who spends a lot of time writing javascript/jquery, I was pretty stunned when my ‘i wonder if this will work’-attempt actually worked.

usort($inbox_messages, function($a, $b){
    return strtotime($b["date"]) - strtotime($a["date"]);
});

Now, is there anything I should know about implementations like that? Performance issues, backwards compatibility, scope etc.

Is this common practice, or bad code?

Answer 1

This feature is called anonymous functions, and it becomes available since PHP 5.3:

Anonymous functions, also known as closures, allow the creation of
functions which have no specified name. They are most useful as the
value of callback parameters, but they have many other uses.

There’s a (slight) difference in scope resolution as well for them:

Closures may also inherit variables from the parent scope. Any such
variables must be declared in the function header. Inheriting
variables from the parent scope is not the same as using global
variables.

So, in this example:

$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);

… from that documentation page both $tax and $total are effectively injected into a closure ($callback) scope without using global keyword.

Anonymous functions are in fact objects of Closure class. And it’s not just an implementation feature: as (since PHP 5.4) these functions can use $this object (similar to a context object in JavaScript), there are some useful methods that can change this context (Closure::bind, Closure::bindTo – compare these with Function.bind, for example).

Speaking more specifically, it’s actually a good practice using anonymous functions when map/filter-ing collection, as these functions won’t go into the global scope. But, of course, it’s not a solution if you need your code to work on PHP 5.2.