PHP Functions

User-Defined Functions, Parameters, & Return Types

Functions are reusable blocks of code that execute when called. PHP 8 functions support scalar type hints, return type declarations, default parameter values, variadic arguments, and named parameters.

Modern PHP Function Signature

function functionName(type $param1, type $param2 = defaultValue): returnType
{
    // Function Body
    return $value;
}

Function Features Architecture

flowchart TD
    A["Function Invocation"] --> B["Validate Argument Types (strict_types=1)"]
    B --> C["Execute Function Logic"]
    C --> D["Validate Return Type"]
    D --> E["Return Result Value to Caller"]

Practical Code Example (PHP 8 Signature)

<?php
declare(strict_types=1);

/**
 * Calculates item totals with tax and optional discount using named parameters.
 */
function calculateInvoiceTotal(
    float $amount,
    float $taxRate = 0.08,
    float $discount = 0.0,
    string ...$notes // Variadic parameter (captures extra trailing string arguments)
): float {
    $taxableAmount = max(0.0, $amount - $discount);
    $total = $taxableAmount + ($taxableAmount * $taxRate);

    if (!empty($notes)) {
        // Process notes
    }

    return round($total, 2);
}

// Invoking function with PHP 8 Named Arguments (Order doesn't matter!)
$finalTotal = calculateInvoiceTotal(
    amount: 250.00,
    discount: 50.00, // Skipped $taxRate to use default 0.08!
    notes: "Applied Spring Coupon", "Expedited Shipping"
);

echo "Final Invoice Total: $" . $finalTotal . "
";

// Arrow Function (Anonymous Closure)
$multiplier = 3;
$numbers = [1, 2, 3, 4];
$tripled = array_map(fn(int $n): int => $n * $multiplier, $numbers);

echo "Tripled Numbers: " . implode(", ", $tripled) . "
";

Best Practices

• Enforce Type Hints & Return Types: Declare types on all function arguments and return types (: int, : string, : void).
• Use PHP 8 Named Arguments for Readability: Pass optional parameters explicitly by name (amount: 100, discount: 10) to eliminate ambiguous boolean flags.
• Use Arrow Functions fn() for Short Closures: Simplifies single-expression closures passed to array_map(), array_filter(), and usort().

Self-Check Challenge

Write a typed function formatCurrency(float $amount, string $currency = 'USD'): string that returns $amount formatted to 2 decimal places prefixed by the currency code!