Functions, Type Declarations & Named Arguments

In Plain English

Functions encapsulate reusable logic. Master typed parameters (int, string, ?array), return types (: void, : string), union types (int|float), variadic parameters (...$items), arrow functions (fn() =>), and PHP 8 named arguments.

The Mental Model

"A typed function is like a certified vending machine slot: it strictly specifies that only a 25-cent coin fits, and guarantees a cold beverage drops out of the dispenser."

Deep Dive: How It Works

Type Signatures: Enforces argument types and return types (e.g. function add(int $a, int $b): int).

Nullable Types: Prefix types with ? (e.g. ?string $name allows string or null).

Union Types (PHP 8): int|float $number allows multiple valid types.

Named Arguments (PHP 8): Call functions by parameter name (formatUser(name: "Alice", active: true)).

Arrow Functions: Concise one-liner closures fn($x) => $x * 2 that automatically capture parent scope variables by value.

Syntax Blueprint

<?php

function calculateTax(float $subtotal, float $rate = 0.08): float {
    return $subtotal * $rate;
}

$tax = calculateTax(subtotal: 100.0, rate: 0.10);

Define parameter and return types with defaults, and invoke with named arguments.

Core Rules to Remember

Type Declarations: Prevents runtime bugs by strictly validating inputs and outputs at function boundaries.

Named Arguments: Enables skipping optional parameters with defaults and clarifies function call semantics.

Variadic Functions: function sum(...$numbers) gathers arbitrary arguments into a single array.

Common Beginner Traps & How to Fix Them

Mistake:Passing null to a non-nullable parameter type (e.g. string $str without ?).

Why it happens: PHP throws a TypeError unless the type is explicitly declared as nullable (?string or string|null).

How to fix: Use ?string when a parameter or return value can legitimately be null.

Pro Tip: Use arrow functions (fn($x) => ...) inside array_map and array_filter for clean, readable pipelines.

Live Interactive Example

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Typed Function with Named Arguments & Arrow Closure

PHP 8.2 / CLI

Output Console

Click "Run Code" to view the rendered output.

How it works: Named argument shout: true bypassed default parameter positions cleanly, and arrow function doubled numbers.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Implement Typed Discount Calculator

Define a function applyDiscount(float $price, float $percent = 10.0): float.
Calculate discount = price * (percent / 100) and return price - discount.
In main script, call applyDiscount(price: 200.0, percent: 15.0) and print "Final Price: $[result]".

Sandbox Output

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