if (empty($_POST['email'])) { $form->addErrorMessage('email', 'Please enter your email address.'); }In this example, if the email field in the form is empty, an error message is added to the form using the addErrorMessage method. The first argument to addErrorMessage is the name of the field to which the message pertains, and the second argument is the actual error message. There are many PHP libraries that provide functionality for working with forms, including some that include support for adding error messages. Some popular libraries include Symfony Forms, Laravel Forms, and Zend Form.