callable-types-rfc-e3.php

<?php declare(strict_types=1);

function crunch_data(array $data, callable(int $left, int $right): int $callback): array {
    $result = [];
    foreach($data as $left => $right) $result[] = $callback($left, $right);

    return $result;
}

$crunched = crunch_data([1 => 2, 3 => 4], function(int $left, int $right): int {
    return $left * $right;
});

var_dump($crunched);