문자 세트에서 특정 크기의 모든 조합을 어떻게 생성할 수 있습니까?

function sampling($chars, $size, $combinations = array()) {
    if (empty($combinations)) {
        $combinations = $chars;
    }
    if ($size == 1) {
        return $combinations;
    }
    $new_combinations = array();
    foreach ($combinations as $combination) {
        foreach ($chars as $char) {
            $new_combinations[] = $combination . $char;
        }
    }
    return sampling($chars, $size - 1, $new_combinations);
}

$output = sampling($chars, 2);
var_dump($output);

array(9) {
  [0]=>
  string(2) "aa"
  [1]=>
  string(2) "ab"
  [2]=>
  string(2) "ac"
  [3]=>
  string(2) "ba"
  [4]=>
  string(2) "bb"
  [5]=>
  string(2) "bc"
  [6]=>
  string(2) "ca"
  [7]=>
  string(2) "cb"
  [8]=>
  string(2) "cc"
}