code: public static function COMBIN($numObjs, $numInSet)
      {
          $numObjs = Functions::flattenSingleValue($numObjs);
          $numInSet = Functions::flattenSingleValue($numInSet);
  
          if ((is_numeric($numObjs)) && (is_numeric($numInSet))) {
              if ($numObjs < $numInSet) {
                  return Functions::NAN();
              } elseif ($numInSet < 0) {
                  return Functions::NAN();
              }
  
              return round(self::FACT($numObjs) / self::FACT($numObjs - $numInSet)) / self::FACT($numInSet);
          }
  
          return Functions::VALUE();
      }
docs: COMBIN.
  
  Returns the number of combinations for a given number of items. Use COMBIN to
  determine the total possible number of groups for a given number of items.
  
  Excel Function:
  COMBIN(numObjs,numInSet)
  
  @category Mathematical and Trigonometric Functions
  
  @param int $numObjs Number of different objects
  @param int $numInSet Number of objects in each combination
  
  @return int Number of combinations