PHP Functions

PHP Functions: Reusable Logic with Strict PHP 8 Types

A PHP function is a named, reusable block of code that accepts input parameters, performs a task, and optionally returns a result. Breaking scripts into small single-purpose functions eliminates duplication and makes your code easier to test.

Defining Typed Functions and Default Arguments

Modern PHP functions declare parameter types and an explicit return type (such as : float, : string, or : void). You can also assign default values to trailing parameters.

<?php
declare(strict_types=1);

function calculateDiscountedPrice(
    float $price,
    float $discountPercent = 10.0,
    bool $roundResult = true
): float {
    $discounted = $price * (1 - ($discountPercent / 100));
    return $roundResult ? round($discounted, 2) : $discounted;
}

// Positional arguments
echo calculateDiscountedPrice(150.0, 20.0) . PHP_EOL;

// PHP 8 Named arguments allow skipping defaults cleanly
echo calculateDiscountedPrice(price: 99.99, roundResult: false) . PHP_EOL;

Named arguments let callers pass parameters by name rather than strict order, making boolean flags and optional settings self-documenting.

Variadic Functions and Arrow Functions

Use the spread operator ... to accept any number of arguments into an array, and use arrow functions (fn() => ...) for concise one-line callbacks.

<?php
declare(strict_types=1);

function sumInvoices(float ...$amounts): float
{
    return array_sum($amounts);
}

$numbers = [2, 4, 6, 8];
$doubled = array_map(fn (int $n): int => $n * 2, $numbers);

echo "Invoice sum: " . sumInvoices(19.5, 40.0, 12.25) . PHP_EOL;
print_r($doubled);

Common Mistakes to Avoid

• Omitting return types: Leaving off : string or : void lets callers pass or receive unexpected null values silently.
• Putting required parameters after optional ones: Required parameters must always appear before parameters with default values.
• Mutating outside variables unexpectedly: Functions operate on copies of scalar values and arrays unless you explicitly pass a parameter by reference (&$var).

Frequently Asked Questions

What does : void mean as a return type?

: void tells PHP that the function performs an action (like printing or saving a file) and does not return any value.

What is the difference between a closure and an arrow function in PHP?

An arrow function (fn($x) => $x * 2) automatically captures variables from the outer scope by value, whereas a regular closure (function ($x) use ($factor) { ... }) requires an explicit use clause.

Can a PHP function return multiple types?

Yes. PHP 8 supports union types like int|float or nullable types like ?string (string|null).