Pure Function¶

A pure function is a function that adheres to two fundamental principles:

Deterministism: for the same input, a pure function will always return the same output. There are no hidden dependencies or side effects that could change the result.
No side-effect: a pure function does not modify any external state or data. It does not change variables outside its scope, write to a database, modify a file, or perform any action that affects the world outside the function.

A function that is not pure is an impure function.

This notion applies to functions, methods, native and custom functions, closures and arrow functions.

<?php

    function add(int $a, int $b): int {
        return $a + $b;
    }

?>