use Symfony\Component\Form\FormInterface; $form = ...; // create your form instance here if ($form instanceof FormInterface) { if ($form->isValid()) { // process form data here } else { // handle invalid form data here } } else { // handle errors here }In this code example, we create a form instance and check if it is an instance of the FormInterface. We then call the isValid function to check if the form is valid or not. If the form is valid, we can process the form data. If it is not valid, we can handle the invalid data appropriately. If there are any errors, we can handle them as well. Package library: The FormInterface isValid function is part of the Symfony Component Form library. This library provides a set of PHP classes that can be used to build and process HTML forms. It is part of the Symfony framework and is available as a separate package that can be installed using Composer.