cmsot

class Mutation
{
    private $len = 0;
    private $arr = [];
    private $table = [];
    private $result = [];

    public function __construct(array $arr, int $len)
    {
        $this->len = $len;
        $this->setHashTable($arr);
        $this->arr = array_unique($arr);
    }

    private function setHashTable(array $arr)
    {
        foreach ($arr as $value) {
            isset($this->table[$value]) ? $this->table[$value] += 1 : $this->table[$value] = 1;
        }
    }

    private function build()
    {
        $this->result = array_reduce(array_fill(0, $this->len - 1, $this->arr), function ($previous, $current) {
            $result = [];
            foreach ($current as $a) {
                foreach ($previous as $b) {
                    if (is_array($b)) {
                        $values = array_count_values($b);
                        if (empty($values[$a]) || $this->table[$a] > $values[$a]) {
                            $result[] = array_merge([$a], $b);
                        }
                    } elseif ($a !== $b || $this->table[$a] > 1) {
                        $result[] = array_merge([$a], [$b]);
                    }
                }
            }

            return $result;
        }, $this->arr);

    }

    public function print()
    {
        empty($this->result) && $this->build();
        print_r($this->result);
    }

    public function getCount()
    {
        empty($this->result) && $this->build();

        return count($this->result);
    }
}

$mutation = new Mutation([1, 2, 3], 2);
$mutation->print();
echo $mutation->getCount();