code: public static function PRODUCT(...$args)
      {
          // Return value
          $returnValue = null;
  
          // Loop through arguments
          foreach (Functions::flattenArray($args) as $arg) {
              // Is it a numeric value?
              if ((is_numeric($arg)) && (!is_string($arg))) {
                  if ($returnValue === null) {
                      $returnValue = $arg;
                  } else {
                      $returnValue *= $arg;
                  }
              }
          }
  
          // Return
          if ($returnValue === null) {
              return 0;
          }
  
          return $returnValue;
      }
docs: PRODUCT.
  
  PRODUCT returns the product of all the values and cells referenced in the argument list.
  
  Excel Function:
  PRODUCT(value1[,value2[, ...]])
  
  @category Mathematical and Trigonometric Functions
  
  @param mixed ...$args Data values
  
  @return float