Symfony 4 PHP Exception DoctrineORMORMException: "Unknown Entity namespace ali as 'User'."

one text

Solution:

You need to import all the dependencies used as User, Request, ViewEvent, KernelEvent, etc.

By the way, is a good practice to import the repository (UserRepository) and not the entityManager, but you dont need it because you have the $user already. You dont need to find it again.

I think this should be enough if you have the user classes in those namespaces (locations):

use Doctrine\ORM\EntityManagerInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Event\ViewEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use ApiPlatform\Core\EventListener\EventPriorities;
use App\Entity\User;

final class RegisterMailSubscriber implements EventSubscriberInterface
{
    private $mailer;

    public function __construct(\Swift_Mailer $mailer)
    {
        $this->mailer = $mailer;
    }

    public static function getSubscribedEvents()
    {
        return [
            KernelEvents::VIEW => ['sendMail', EventPriorities::POST_WRITE],
        ];
    }

    public function sendMail(ViewEvent $event): void
    {
        $user = $event->getControllerResult(); 
        $method = $event->getRequest()->getMethod();

        if (!$user instanceof User || Request::METHOD_POST !== $method) {
            return;
        }

       $userEmail = $user->getEmail(); //for example. You got the user 5 lines before.
        
    }
}