Arrow Functions

Arrow functions were introduced in PHP 7.4 as a more concise syntax for anonymous functions.

Both anonymous functions and arrow functions are implemented using the Closure class.

Arrow functions have the basic form fn (argument_list) => expr.

Arrow functions support the same features as anonymous functions, except that using variables from the parent scope is always automatic.

When a variable used in the expression is defined in the parent scope it will be implicitly captured by-value. In the following example, the functions $fn1 and $fn2 behave the same way.

<?php

$y = 1;
 
$fn1 = fn($x) => $x + $y;
// equivalent to using $y by value:
$fn2 = function ($x) use ($y) {
    return $x + $y;
};

var_export($fn1(3));
?>

4

This also works if the arrow functions are nested:

<?php

$z = 1;
$fn = fn($x) => fn($y) => $x * $y + $z;
// Outputs 51
var_export($fn(5)(10));
?>

Similarly to anonymous functions, the arrow function syntax allows arbitrary function signatures, including parameter and return types, default values, variadics, as well as by-reference passing and returning. All of the following are valid examples of arrow functions:

<?php

fn(array $x) => $x;
static fn($x): int => $x;
fn($x = 42) => $x;
fn(&$x) => $x;
fn&($x) => $x;
fn($x, ...$rest) => $rest;

?>

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. Anonymous functions can be used instead for by-ref bindings.

<?php

$x = 1;
$fn = fn() => $x++; // Has no effect
$fn();
var_export($x);  // Outputs 1

?>