05 Functions

Goal

Write functions with typed parameters and return types, understand default arguments, and use arrow functions for short callbacks.

Prerequisites

• Control Flow

Basic Functions

<?php

function greet(string $name): string {
    return "Hello, {$name}!";
}

echo greet("Mei") . "\n";

The string $name and : string are optional type declarations — PHP will throw a TypeError at call time if you pass the wrong type, rather than silently coercing. This is the gradual-typing system mentioned in 03 Variables and Types.

Default Arguments

<?php

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

echo greet("Mei") . "\n";           // Hello, Mei!
echo greet("Mei", "Hi") . "\n";     // Hi, Mei!

Named Arguments (PHP 8+)

<?php

echo greet(name: "Mei", greeting: "Hey") . "\n"; // Hey, Mei!
echo greet(greeting: "Yo", name: "Mei") . "\n";  // order doesn't matter with named args

Arrow Functions

For short, single-expression callbacks, fn is more compact than a full function and automatically captures outer variables (no use clause needed):

<?php

$multiplier = 3;
$triple = fn(int $n): int => $n * $multiplier;

echo $triple(4) . "\n"; // 12

Compare to the older closure syntax, which needs an explicit use:

<?php

$triple = function (int $n) use ($multiplier): int {
    return $n * $multiplier;
};

Prefer fn for anything that fits on one line; reach for function ... use (...) when the body needs multiple statements.

Checkpoint

<?php

function double(int $n): int {
    return $n * 2;
}

$numbers = [1, 2, 3, 4];
$doubled = array_map(fn($n) => double($n), $numbers);

print_r($doubled);

Expected output:

Array
(
    [0] => 2
    [1] => 4
    [2] => 6
    [3] => 8
)

Next

Continue to 06 Arrays.