code: protected function getClassForOption($index)
      {
          // If true we need another offset.
          $intSub   = ($this->arrConfiguration['includeBlankOption'] ? -1 : 1);
          $strClass = $this->strName;
  
          if ($index == 0) {
              $strClass .= ' first';
          } elseif ($index === (count($this->options) - $intSub)) {
              $strClass .= ' last';
          }
  
          if (($index % 2) == 1) {
              $strClass .= ' even';
          } else {
              $strClass .= ' odd';
          }
  
          return ((strlen($this->strClass)) ? ' ' . $this->strClass : '') . $strClass;
      }
docs: Get the css class for an option.
  
  @param int $index The sequence number of the current option.
  
  @return string