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);
}