code: public static function ERF($lower, $upper = null)
      {
          $lower = Functions::flattenSingleValue($lower);
          $upper = Functions::flattenSingleValue($upper);
  
          if (is_numeric($lower)) {
              if ($upper === null) {
                  return self::erfVal($lower);
              }
              if (is_numeric($upper)) {
                  return self::erfVal($upper) - self::erfVal($lower);
              }
          }
  
          return Functions::VALUE();
      }
docs: ERF.
  
  Returns the error function integrated between the lower and upper bound arguments.
  
  Note: In Excel 2007 or earlier, if you input a negative value for the upper or lower bound arguments,
  the function would return a #NUM! error. However, in Excel 2010, the function algorithm was
  improved, so that it can now calculate the function for both positive and negative ranges.
  PhpSpreadsheet follows Excel 2010 behaviour, and accepts nagative arguments.
  
  Excel Function:
  ERF(lower[,upper])
  
  @param float $lower lower bound for integrating ERF
  @param float $upper upper bound for integrating ERF.
  If omitted, ERF integrates between zero and lower_limit
  
  @return float