Learn

Functions use the function keyword. Type declarations (since 7.0+ for params, 7.1+ for returns) are optional but highly recommended:

function square(int $n): int {
    return $n * $n;
}

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

echo square(5);                  // 25
echo greet("Alice");              // Hello, Alice!
echo greet("Bob", "Howdy");      // Howdy, Bob!

Functions are case-insensitive (legacy quirk — call Square(5) and it works). Function names live in their own global namespace.

Add declare(strict_types=1); at the top of a file to make type declarations strict — passing a wrong type becomes a TypeError instead of a silent coerce. Recommended for new code.

Variadic params with ...: function sum(...$nums) { return array_sum($nums); }.