Александр Измайлов

class CombinationsCalculator
{
    protected $result;
    protected $combination;

    public function getCombinations($array, $group) {
        $this->result = [];
        $this->combination = [];
        $this->iteration(0, $group, $array, count($array));
        return $this->result;
    }

    protected function iteration($start, $step, $array, $n) {
        if ($step == 0) {
            array_push($this->result,$this->combination);
        }
        else {
            for ($i = $start; $i <= $n - $step; ++$i) {
                array_push($this->combination, $array[$i]);
                $this->iteration($i + 1, $step - 1, $array, $n);
                array_pop($this->combination);
            }
        }
    }
}

$calculator = new CombinationsCalculator();
$result = $calculator->getCombinations([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15], 8);