code: public static function EFFECT($nominal_rate = 0, $npery = 0)
      {
          $nominal_rate = Functions::flattenSingleValue($nominal_rate);
          $npery = (int) Functions::flattenSingleValue($npery);
  
          // Validate parameters
          if ($nominal_rate <= 0 || $npery < 1) {
              return Functions::NAN();
          }
  
          return pow((1 + $nominal_rate / $npery), $npery) - 1;
      }
docs: EFFECT.
  
  Returns the effective interest rate given the nominal rate and the number of
  compounding payments per year.
  
  Excel Function:
  EFFECT(nominal_rate,npery)
  
  @category Financial Functions
  
  @param float $nominal_rate Nominal interest rate
  @param int $npery Number of compounding payments per year
  
  @return float