What functions are considered pure?

Pure function:

• Always returns the same result given the same input arguments.
• Has no side effects (does not modify global state, does not perform input/output, does not modify arguments passed by reference).

Examples:

<?php
// Pure function
function sum(int $a, int $b): int
{
    return $a + $b;
}

// Impure function (modifies global state)
$counter = 0;
function incrementCounter(): void
{
    global $counter;
    $counter++;
}

// Impure function (performs output)
function printMessage(string $message): void
{
    echo $message;
}