Anonymous Functions and Closures

PHP treats functions as values you can store in a variable, pass as an argument, or return from another function — starting with anonymous functions.

Anonymous functions

<?php
$greet = function ($name) {
    return "Hello, $name!";
};

echo $greet("Amara");

An anonymous function (also called a closure) has no name of its own — it's assigned directly to a variable, which is then called just like a named function. This is the shape most callbacks take when passed to functions like array_map() or usort().

Capturing outer variables with use

<?php
$taxRate = 0.08;

$addTax = function ($price) use ($taxRate) {
    return $price * (1 + $taxRate);
};

echo $addTax(100); // 108

By default, an anonymous function can't see variables from the surrounding scope — use ($taxRate) explicitly imports it. Without use, referencing $taxRate inside the closure would trigger an "undefined variable" warning, even though it clearly exists just outside.

Capturing by reference

<?php
$total = 0;

$addToTotal = function ($amount) use (&$total) {
    $total += $amount;
};

$addToTotal(10);
$addToTotal(25);

echo $total; // 35

use (&$total) captures $total by reference instead of by value — changes made inside the closure persist in the outer variable after the closure returns. Without the &, each call would modify its own private copy, and the outer $total would still be 0.

Arrow functions: a shorter syntax

<?php
$taxRate = 0.08;
$addTax = fn($price) => $price * (1 + $taxRate);

echo $addTax(100); // 108

Arrow functions (fn(...) => expression) automatically capture any outer variable they reference — no use needed — but are limited to a single expression, whose result is returned implicitly. They're a good fit for the short, one-line callbacks passed to array_map(), array_filter(), and similar functions; reach for a full anonymous function when the logic needs more than one statement.

Passing a closure as a callback

<?php
$numbers = [5, 3, 8, 1];

usort($numbers, fn($a, $b) => $a <=> $b);

print_r($numbers); // [1, 3, 5, 8]

usort() sorts an array using a custom comparison function — here, the spaceship operator <=> returns -1, 0, or 1 depending on whether $a is less than, equal to, or greater than $b, which is exactly what a comparison callback is expected to return. This pattern — a built-in function taking a closure to customize its behavior — is everywhere in PHP's standard library.