/** http://docstore.mik.ua/orelly/webprog/pcook/ch04_26.htm */
function permutations($items, $perms = []) {
    if (empty($items)) {
        yield $perms;
    } else {
        for ($i = count($items) - 1; $i >= 0; --$i) {
            $newitems = $items;
            $newperms = $perms;
            list($foo) = array_splice($newitems, $i, 1);
            array_unshift($newperms, $foo);
            yield from permutations($newitems, $newperms);
        }
    }
}