code: public static function addCssClass(&$options, $class)
      {
          if (isset($options['class'])) {
              if (is_array($options['class'])) {
                  $options['class'] = self::mergeCssClasses($options['class'], (array) $class);
              } else {
                  $classes = preg_split('/\s+/', $options['class'], -1, PREG_SPLIT_NO_EMPTY);
                  $options['class'] = implode(' ', self::mergeCssClasses($classes, (array) $class));
              }
          } else {
              $options['class'] = $class;
          }
      }
docs: Adds a CSS class (or several classes) to the specified options.
  
  If the CSS class is already in the options, it will not be added again.
  If class specification at given options is an array, and some class placed there with the named (string) key,
  overriding of such key will have no effect. For example:
  
  ```php
  $options = ['class' => ['persistent' => 'initial']];
  Html::addCssClass($options, ['persistent' => 'override']);
  var_dump($options['class']); // outputs: array('persistent' => 'initial');
  ```
  
  @param array $options the options to be modified.
  @param string|array $class the CSS class(es) to be added
  @see mergeCssClasses()
  @see removeCssClass()