code: public static function COUNTBLANK(...$args)
      {
          $returnValue = 0;
  
          // Loop through arguments
          $aArgs = Functions::flattenArray($args);
          foreach ($aArgs as $arg) {
              // Is it a blank cell?
              if (($arg === null) || ((is_string($arg)) && ($arg == ''))) {
                  ++$returnValue;
              }
          }
  
          return $returnValue;
      }
docs: COUNTBLANK.
  
  Counts the number of empty cells within the list of arguments
  
  Excel Function:
  COUNTBLANK(value1[,value2[, ...]])
  
  @category Statistical Functions
  
  @param mixed ...$args Data values
  
  @return int