PHP Closures: Functions That Remember Their Surroundings

A closure is a function that carries some outside variables with it. In PHP, every anonymous function is an object of the Closure class, and you pick the outside variables with use.

index.php

PHP

<?php
$greeting = "Hello";

$greet = function (string $name) use ($greeting): string {
    return "$greeting, $name!";
};

echo $greet("Asha");

Output

OUTPUT

Hello, Asha!

Without use ($greeting), the function could not see $greeting, because functions in PHP have their own scope.

The value is copied when the closure is created

index.php

PHP

<?php
$count = 1;

$show = function () use ($count) {
    echo "Count is $count\n";
};

$count = 99;   // changed after the closure was made

$show();

Output

OUTPUT

Count is 1

The closure kept the value 1.

use by reference

Add & to see later changes, and to let the closure change the outside variable.

index.php

PHP

<?php
$total = 0;

$add = function (int $n) use (&$total): void {
    $total += $n;
};

$add(5);
$add(10);

echo $total;

Output

OUTPUT

A counter factory

index.php

PHP

<?php
function makeCounter(): callable
{
    $count = 0;

    return function () use (&$count): int {
        return ++$count;
    };
}

$a = makeCounter();
$b = makeCounter();

echo $a() . $a() . $a() . " ";
echo $b();

Output

OUTPUT

123 1

Each counter has its own private $count. That is the main power of closures: private state without a class.

Closures inside classes

A closure created inside a method automatically has access to $this.

index.php

PHP

<?php
class Cart
{
    private array $items = ["Pen", "Book"];

    public function printer(): callable
    {
        return function (): void {
            echo count($this->items) . " items";
        };
    }
}

$print = (new Cart())->printer();
$print();

Output

OUTPUT

2 items

Static closures

If a closure does not need $this, mark it static. It will not keep the object alive.

index.php

PHP

<?php
$double = static fn(int $n): int => $n * 2;

echo $double(21);

Output

OUTPUT

Closure vs arrow function

	`function () use (...)`	`fn() =>`
Outside variables	Listed with `use`	Captured automatically
Body	Many statements	One expression
By reference	Yes, with `&`	No

info

Why this matters in WordPress

WordPress hooks often take a closure, as in add_action("init", function () { ... });. You will see that in WordPress Hooks.