PHP Functions

<?php
function greet() {
    echo "Hello!\n";
}

greet();        // Hello!

// With parameters and return
function add($a, $b) {
    return $a + $b;
}

echo add(3, 4); // 7

<?php
declare(strict_types=1);

function divide(int $a, int $b): float {
    return $a / $b;
}

// Nullable type
function findUser(?int $id): ?User { /* ... */ }

// Union type (PHP 8.0+)
function format(int|string $value): string {
    return (string) $value;
}

// Intersection type (PHP 8.1+) - object must implement BOTH
function process(Iterator&Countable $data) { /* ... */ }

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

echo greet();                       // Hello, Guest!
echo greet("Ruban");                // Hello, Ruban!
echo greet("Ruban", "Welcome");     // Welcome, Ruban!

// Defaults must come after required params
function bad(string $a = "x", string $b) {}  // ERROR
function ok(string $b, string $a = "x") {}   // OK

<?php
function isAdult(int $age): bool {
    return $age >= 18;
}

// void - function returns nothing
function log(string $msg): void {
    file_put_contents("app.log", $msg, FILE_APPEND);
}

// never - function never returns (throws or exits)
function abort(string $msg): never {
    throw new RuntimeException($msg);
}

// static - returns instance of the same class (PHP 8.0+)
class Builder {
    public function chain(): static { return $this; }
}

<?php
// Variadic - collect all extra args into an array
function sum(int ...$numbers): int {
    return array_sum($numbers);
}

echo sum(1, 2, 3);              // 6
echo sum(1, 2, 3, 4, 5, 6);     // 21

// Spread - expand an array into individual args
$nums = [10, 20, 30];
echo sum(...$nums);             // 60

// Combine fixed + variadic
function logEvent(string $level, string ...$messages): void {
    foreach ($messages as $msg) {
        echo "[$level] $msg\n";
    }
}

<?php
function createUser(
    string $name,
    string $email,
    string $role = "user",
    bool $active = true,
    ?string $department = null,
) { /* ... */ }

// Without named args - what does each value mean?
createUser("Ruban", "ruban@x.com", "admin", false, "Eng");

// With named args - intent crystal clear
createUser(
    name: "Ruban",
    email: "ruban@x.com",
    role: "admin",
    active: false,
    department: "Eng",
);

// Skip middle defaults
createUser(name: "Ruban", email: "ruban@x.com", department: "Eng");

<?php
$nums = [1, 2, 3, 4, 5];

// Arrow function - auto-captures outer variables
$tax = 0.10;
$withTax = array_map(fn($n) => $n + $n * $tax, $nums);

// Equivalent classic closure
$withTax = array_map(function($n) use ($tax) {
    return $n + $n * $tax;
}, $nums);

// Sort callback
usort($users, fn($a, $b) => $a->age <=> $b->age);

// Filter
$adults = array_filter($users, fn($u) => $u->age >= 18);

<?php
// Closure stored in a variable
$multiply = function ($a, $b) {
    return $a * $b;
};

echo $multiply(3, 4);   // 12

// Capture outer variable by value
$prefix = "user_";
$prefixer = function ($name) use ($prefix) {
    return $prefix . $name;
};
echo $prefixer("alice");  // user_alice

// Capture by reference - mutations visible to outer
$count = 0;
$increment = function () use (&$count) {
    $count++;
};
$increment(); $increment(); $increment();
echo $count;              // 3

// Capture multiple
$header = "App: ";
$debug  = true;
$log = function ($msg) use ($header, $debug) {
    if ($debug) echo $header . $msg;
};

<?php
// Old way - pass functions by name string
$upper = array_map("strtoupper", $words);

// PHP 8.1+ - first-class callable syntax
$upper = array_map(strtoupper(...), $words);

// Works with methods too
$callbacks = [
    $user->save(...),
    $logger->info(...),
    User::create(...),
];

// IDE can verify these now - typo-safe and refactorable

<?php
// Factorial
function factorial(int $n): int {
    return $n <= 1 ? 1 : $n * factorial($n - 1);
}
echo factorial(5);     // 120

// Walk a directory tree
function listFiles(string $dir): array {
    $files = [];
    foreach (scandir($dir) as $entry) {
        if ($entry === "." || $entry === "..") continue;
        $path = "$dir/$entry";
        if (is_dir($path)) {
            $files = array_merge($files, listFiles($path));
        } else {
            $files[] = $path;
        }
    }
    return $files;
}