14. PHP Functions

What you will learn

• How to create and call a function
• Parameters, return values, and default values
• Type declarations
• Variable scope and arrow functions

What is a function?

A function is a named block of code you can run whenever you need it. PHP has thousands of built-in functions, such as strlen(), and you can write your own.

Creating and calling

<?php
function say_hello() {
    echo "Hello from a function!";
}

say_hello();

Declaring a function does nothing by itself. It runs only when you call it with brackets.

Parameters and return values

Parameters are the inputs listed in the declaration. return sends a value back and ends the function.

<?php
function add($a, $b) {
    return $a + $b;
}

$total = add(5, 7);
echo "5 + 7 = $total";

Default values

<?php
function welcome($name = "friend") {
    return "Welcome, $name!";
}

echo welcome("Sam") . "<br>";
echo welcome();

Type declarations

You can say what type each parameter and the return value should be. PHP then catches mistakes early.

<?php
declare(strict_types=1);

function area(float $width, float $height): float {
    return $width * $height;
}

echo area(6, 4.5);

With declare(strict_types=1); as the first statement of the file, PHP refuses wrong types, such as passing text where a number is required, instead of silently converting. Type declarations make code safer and easier to read. A ?string type means “a string or null”, and void means “returns nothing”.

Named arguments

In PHP 8 you can pass arguments by name, in any order, which makes calls easier to read.

<?php
function box(int $width, int $height, string $color = "blue"): string {
    return "$width x $height in $color";
}

echo box(height: 20, width: 10);

Returning several values

A function returns one value, but that value can be an array.

<?php
function min_max(array $numbers): array {
    return [min($numbers), max($numbers)];
}

[$low, $high] = min_max([4, 9, 1, 7]);
echo "Low: $low, High: $high";

Scope

A variable made inside a function exists only there, and outside variables are not visible inside. Pass what you need in as parameters. Avoid the global keyword, which hides where values come from.

<?php
$rate = 0.2;

function with_tax(float $price, float $rate): float {
    return $price * (1 + $rate);
}

echo with_tax(100, $rate);

Arrow functions

For short one-line functions, PHP has arrow functions with fn. They can read outside variables automatically.

<?php
$factor = 3;
$triple = fn($n) => $n * $factor;

echo $triple(5) . "<br>";

$numbers = [1, 2, 3];
$doubled = array_map(fn($n) => $n * 2, $numbers);
echo implode(", ", $doubled);

array_map() applies a function to every item and returns a new array.

Common mistakes

• Declaring a function and never calling it.
• Forgetting return and getting null.
• Expecting outside variables to be visible inside a function.
• Declaring the same function name twice, which is a fatal error.
• Making one huge function that does many unrelated jobs.

<?php
function rectangle_area($width, $height) {
    return $width * $height;
}

echo "Area: " . rectangle_area(6, 4);

Summary

• Functions package reusable code, with parameters in and return out.
• Use type declarations and defaults, and pass values in rather than using globals.
• Arrow functions suit short callbacks.