Functions and typed boundaries

The idea

A function packages a transformation behind parameters and a return value. Echo writes output; return provides data to the caller and ends the invocation. Parameter and return declarations describe accepted values, but scalar coercion behavior also depends on strict_types. A strict declaration belongs at the top of a PHP file and affects calls made from that file rather than changing every caller in the application.

Ordinary parameters do not give a function permission to rebind the caller’s variable. An ampersand parameter explicitly requests reference behavior and should be used only when changing that variable is part of the contract. Objects have their own identity considerations, so distinguish replacing a variable from changing an object’s properties. Prefer returning a new result for transformations that callers need to reason about independently.

<?php
declare(strict_types=1);
function subtotal(int $quantity, int $price): int {
    return $quantity * $price;
}
echo subtotal(3, 7), "
";

21

The strict calling file supplies two integers, and the function returns their product. Echo displays the returned result outside the calculation.