public function sendVerificationLink(array $credentials, Closure $callback = null)
{
$user = $this->getUser($credentials);
if (is_null($user)) {
return Contracts\VerificationBroker::INVALID_USER;
}
$token = $this->tokens->create($user);
$this->emailVerificationLink($user, $token, $callback);
return Contracts\VerificationBroker::VERIFICATION_LINK_SENT;
}