flatMap.php

<?php

function is_iterable($value) {
    return is_array($value) || $value instanceof Traversable;
}

function flatMap(/* iterable */ $iterable, callable $callable) {
    foreach ($iterable as $value) {
        if (is_iterable($value)) {
            foreach (flatMap($value, $callable) as $next) {
                yield $next;
            }
        } else {
            yield $callable($value);
        }
    }
}

$callable = function($value) {
    print "value: {$value}\n";
    return $value * 2;
};

$one = [1, 2, 3];

print_r(
    iterator_to_array(
        flatMap($one, $callable)
    )
);

$two = [1, [2, 3], 4];

print_r(
    iterator_to_array(
        flatMap($two, $callable)
    )
);

$three = [1, [2, 3, new ArrayIterator([4])], 5];

print_r(
    iterator_to_array(
        flatMap($three, $callable)
    )
);

Thanks for the tip. In that case, I definitely prefer not to use yield from. I don't think the relatively small performance hit is worth the caveat of duplicate indices.