Use variable by reference in arrow function

Question

PHP 7.4 introduced Arrow functions. And it also introduced implicit by-value scope binding which eliminate the need for use keyword.

Now if we want to use a variable out of a closure's scope by reference with a regular anonymous function we would do this:

$num = 10;
call_user_func(function() use (&$num) {
$num += 5;
});
echo $num; // Output: 15

But using an arrow function it seems impossible

$num = 10;
call_user_func(fn() => $num += 5);

echo $num; // Output: 10

So how to use $num variable by reference?

Answer 1

Reading the documentation about it, it says...

By-value variable binding
As already mentioned, arrow functions use by-value variable binding. This is roughly equivalent to performing a use($x) for every variable $x used inside the arrow function. A by-value binding means that it is not possible to modify any values from the outer scope:

$x = 1; 
$fn = fn() => $x++; // Has no effect 
$fn(); 
var_dump($x); // int(1)

So it is not possible ATM.

Answer 2

You can use an object to do this.

$state = new stdClass; 
$state->index = 1;

$fn = fn() => $state->index++; // Now, has effect.
$fn();

var_dump($x); // int(2)

It works because objects always pass by reference. I don't encourage you to use this but in some places it's useful.