Simple flatmap() or mapcat() for PHP 7

flatmap.php

<?php

/**
 * If you need something safer or more complete see https://github.com/lstrojny/functional-php,
 * in particular, https://github.com/lstrojny/functional-php/blob/master/src/Functional/FlatMap.php
 *
 * @param Callable $fn    Mapping function that returns an array
 * @param array    $array Data over which $fn will be mapped
 * @return array
 */
function flatmap(callable $fn, $array)
{
    return array_merge(...array_map($fn, $array));
}

empty($array) === true will create
PHP Warning: array_merge() expects at least 1 parameter, 0 given in ...

due to the fact that splat (...) of empty array evaluates to nothing.

return array_merge([], ...array_map($fn, $array)); will return [], if this is a more desirable behaviour.