code: private static function mergeCssClasses(array $existingClasses, array $additionalClasses)
      {
          foreach ($additionalClasses as $key => $class) {
              if (is_int($key) && !in_array($class, $existingClasses)) {
                  $existingClasses[] = $class;
              } elseif (!isset($existingClasses[$key])) {
                  $existingClasses[$key] = $class;
              }
          }
  
          return array_unique($existingClasses);
      }
docs: Merges already existing CSS classes with new one.
  This method provides the priority for named existing classes over additional.
  @param array $existingClasses already existing CSS classes.
  @param array $additionalClasses CSS classes to be added.
  @return array merge result.
  @see addCssClass()