Incorrect password encryption - symfony

Question

I have a small password encryption problem ^^
in my database the passwords are present, and have to present, in this form:
VwBybV5ATQ9RkdvvVZNOlldEEDU9tDjttju7t8l+HeVe4nskHeMpbuCoQsqqORUQKZ1pg7gGtFocpkSIw8N9kA==
and right now I have a function that needs to generate a password for me. Unfortunately the encryption is not good, because that's what I get:
YmY1NzFkM2VkODYwOGQ1OWFlMTRiZDVkOTc3ZDFkNzQ0ODIzN2U5NWMzNzU0ZjI1Y2U4MTZhYzBiYmExYWJjZTg2Y2JjNzYyM2QwYTJmMDUwYWJiMzQxMjliYjBjYWQxMGZiMzliYzk3OGQwZjYxMGU3Y2E0NjE0ZTkxYzFiYmM=
my code :
public function lostpasswordAction(Request $request)
{
$success = '';
$string = '';
$characters = 'abcdefghijklmnopqrstuvwxyz0123456789';
$max = strlen($characters) - 1;
for ($i = 0; $i < 12; $i++) {
$string .= $characters[mt_rand(0, $max)];
}
if ($request->request->get('email') !== null) {
$em = $this->getDoctrine()->getManager();
$user = $em->getRepository('ApplicationSonataUserBundle:User')->findByEmail($request->request->get('email'));
if (is_null($user)) {
$response = new JsonResponse('Not Found');
$response->setStatusCode(Response::HTTP_NOT_FOUND);
return $response;
}
$login = $user[0]->getEmail();
$password = $string;
$user[0]->setPassword(hash('sha512',$password));
$em->persist($user[0]);
$em->flush();
$message = \Swift_Message::newInstance()
->setSubject('Subject')
->setFrom('no-reply#noreply.com')
->setTo($request->request->get('email'))
->setBody(
$this->renderView(
'emails/lostpassword.txt.twig',
array(
'login' => $login,
'password' => $password
)
),
'text/plain'
);
$return = $this->get('mailer')->send($message);
$success = 'Email sent';
return new JsonResponse($success);
}
$response = new JsonResponse('POST only');
$response->setStatusCode(Response::HTTP_BAD_REQUEST);
return $response;
}
Can someone help me so I get the right shape please?
Thank you in advance