How to find all possible combinations of n by k?

Question

PHP

Formula that counts the number of possible combinations https://www.matburo.ru/tvart_sub.php?p=calc_C
We pass the number of elements (in the array) and the number in the group (how many elements from the total array will be in one subarray).
The task is as follows: we pass the number of elements in the array 15 pieces (unique) and the number in the group, for example 8, after which the script must combine all the options from the common array and form subarrays with the number of elements as specified, 8 pieces each.

The code

//Вначале на простом
$ar = [1,2,3,4,5];
$group = 3;
/* Результат должен получиться следующий
Array
(
    [0] => [1,2,3],
    [1] => [1,2,4],
    [2] => [1,2,5],
    [3] => [1,3,4],
    [4] => [1,3,5],
    [5] => [1,4,5],
    [6] => [2,3,4],
    [7] => [2,3,5],
    [8] => [2,4,5],
    [9] => [3,4,5]
)

С небольшим набором все кажется просто, но что если мы поменяем входные параметры, см.ниже
*/

//Например есть массив
$ar = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15];
$group = 8;
//В результате должен получиться массив с 6435 подмассивов в котором группы по 8 уникальных элементов. В одном подмассиве элементы не могут повторяться.

Answer 1

An example using a class.

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);

Answer 2

function combine($arr, $k) {
   $combs = [];
   $comb = range(0, $k - 1);
   
   while($comb) {
      $combs[] = array_map(function($i) use($arr) {return $arr[$i];}, $comb);
      
      for($i = $k - 1, $max = count($arr) - 1; $i > -1; $i--, $max--) {
         if($comb[$i] < $max) {
            $comb[$i] ++;
            for($j = $i + 1; $j < $k; $j++) {
               $comb[$j] = $comb[$j - 1] + 1;
            }
            break;
         } 
      }
      if($i < 0) $comb = null;
   }
   
   return $combs;
}

print_r(combine([1,2,3,4,5], 3));