class IndexController extends Phalcon\Mvc\Controller
{
    public function initialize()
    {
        \Phalcon\Tag::appendTitle(' - domain.com');
    }
    public function indexAction()
    {
        \Phalcon\Tag::setTitle('Homepage');
    }
}In effect in index.phtml <?php echo Tag::getTitle() ?>
display only Homepage as a title.
Better could be if appendTitle,setTitle,prependTitle use internal indexed array for holding, and Tag::getTtitle() display sorted concatented this internal array.