CraftCMS 3 - Extending the validation on the public user registration form

craft_validate_public_user_registration.php

<?php

use Craft;
use craft\base\Element;
use craft\elements\User;
use craft\events\ModelEvent;

use yii\base\Event;


// Place the following lines in the init() function of a module or a plugin.

$request = Craft::$app->getRequest();

Event::on(
    User::class,
    Element::EVENT_BEFORE_VALIDATE,
    function (Event $event) use($request) {

        // If a user already logged in we are not on the public registration page
        if (Craft::$app->getUser()->getIdentity())
            return;
        
        // Do the custom validation
        if (!$event->sender->firstName) {
            $event->sender->addError('firstName', Craft::t('yii', '{attribute} cannot be blank.', [ 'attribute' => Craft::t('app', 'First Name') ]));
            $event->isValid = false;
        }

        if (!$event->sender->lastName) {
            $event->sender->addError('lastName', Craft::t('yii', '{attribute} cannot be blank.', [ 'attribute' => Craft::t('app', 'Last Name') ]));
            $event->isValid = false;
        }

        if (!$event->sender->email) {
            $event->sender->addError('email', Craft::t('yii', '{attribute} cannot be blank.', [ 'attribute' => Craft::t('app', 'Email') ]));
            $event->isValid = false;
        }

        if ($request->post('userConfirm') != 1) {
            Craft::$app->getSession()->setFlash('userConfirm', 'Ticking that field is required.');
            $event->isValid = false;
        }

    }
);