code: private function getSubIdentifiersRecursively($array, $identifiers)
      {
          $identifier = array_shift($identifiers);
          if (count($identifiers) > 0) {
              $result = [];
  
              foreach ($array[$identifier] as $key => $value) {
                  $result[] = $this->getSubIdentifiersRecursively(
                      $array[$identifier][$key],
                      $identifiers
                  );
              }
  
              return array_merge(...$result);
          }
  
          return array_keys($array[$identifier]);
      }
docs: Calls itself recursively to return identifiers from nested arrays
  
  @param $array
  @param $identifiers
  @return array