code: public static function DISC($settlement, $maturity, $price, $redemption, $basis = 0)
      {
          $settlement = Functions::flattenSingleValue($settlement);
          $maturity = Functions::flattenSingleValue($maturity);
          $price = Functions::flattenSingleValue($price);
          $redemption = Functions::flattenSingleValue($redemption);
          $basis = Functions::flattenSingleValue($basis);
  
          //    Validate
          if ((is_numeric($price)) && (is_numeric($redemption)) && (is_numeric($basis))) {
              $price = (float) $price;
              $redemption = (float) $redemption;
              $basis = (int) $basis;
              if (($price <= 0) || ($redemption <= 0)) {
                  return Functions::NAN();
              }
              $daysBetweenSettlementAndMaturity = DateTime::YEARFRAC($settlement, $maturity, $basis);
              if (!is_numeric($daysBetweenSettlementAndMaturity)) {
                  //    return date error
                  return $daysBetweenSettlementAndMaturity;
              }
  
              return (1 - $price / $redemption) / $daysBetweenSettlementAndMaturity;
          }
  
          return Functions::VALUE();
      }
docs: DISC.
  
  Returns the discount rate for a security.
  
  Excel Function:
  DISC(settlement,maturity,price,redemption[,basis])
  
  @category Financial Functions
  
  @param mixed $settlement The security's settlement date.
  The security settlement date is the date after the issue
  date when the security is traded to the buyer.
  @param mixed $maturity The security's maturity date.
  The maturity date is the date when the security expires.
  @param int $price The security's price per $100 face value
  @param int $redemption The security's redemption value per $100 face value
  @param int $basis The type of day count to use.
  0 or omitted    US (NASD) 30/360
  1                Actual/actual
  2                Actual/360
  3                Actual/365
  4                European 30/360
  
  @return float