fn.php

/**
 * Apply a function which produces an array to all of an array's elements and concatenate the results.
 *
 * @param callable $func The function to be applied to every of $arr's elements to produce a new array each.
 * @param array $arr
 * @return array
 */
function array_flatmap(callable $func, array $arr) : array
{
    return array_merge([], ...array_map($func, $arr));
}

/**
 * Create a list of all possible combinations of input arrays.
 *
 * Example:
 * array_combinations(['a', 'b'], ['c', 'd']) == [['a', 'c'], ['a', 'd'], ['b', 'c'], ['b', 'd']]
 *
 * @param array ...$args
 * @return array
 */
function array_combinations(array ...$args) : array
{
    $array_combinations_ = function ($combinations, $args) use (&$array_combinations_) {
        if (count($args) === 0) {
            return $combinations;
        } else {
            return $array_combinations_(array_flatmap(function ($a) use ($combinations) {
                return array_map(function ($b) use ($a) {
                    return array_merge($b, [$a]);
                }, $combinations);
            }, $args[0]), array_slice($args, 1));
        }
    };

    if (count($args) === 0) {
        return [];
    } else {
        return $array_combinations_([[]], $args);
    }
}

/**
 * Check if $needle is in array $haystack using a custom comparison function, $cmp.
 *
 * @param mixed $needle
 * @param array $haystack
 * @param callable $cmp
 * @return bool
 */
function in_array_f($needle, array $haystack, callable $cmp) : bool
{
    foreach ($haystack as $elem) {
        if ($cmp($needle, $elem)) {
            return true;
        }
    }

    return false;
}