new StringType('username', 32),
      'email' => new StringType('email', 64),
    ));
    $this->csrfTokenRequired = true;
    $this->loginRequired = true;
    $this->requiredGroup = USER_GROUP_ADMIN;
  }
  public function execute($values = array()) {
    if(!parent::execute($values)) {
      return false;
    }
    $username = $this->getParam('username');
    $email = $this->getParam('email');
    if (!$this->userExists($username, $email) || !$this->success) {
        return false;
    }
    //add to DB
    $token = generateRandomString(36);
    $valid_until = (new DateTime())->modify("+48 hour");
    $sql = $this->user->getSQL();
    $res = $sql->insert("UserInvitation", array("username", "email", "token", "valid_until"))
        ->addRow($username, $email, $token, $valid_until)
        ->execute();
    $this->success = ($res !== FALSE);
    $this->lastError = $sql->getLastError();
    //send validation mail
    if($this->success) {
        $request = new SendMail($this->user);
        $link = "http://localhost/acceptInvitation?token=$token";
        $this->success = $request->execute(array(
            "from" => "webmaster@romanh.de",
            "to" => $email,
            "subject" => "Account Invitation for web-base@localhost",
            "body" =>
"Hello,
you were invited to create an account on web-base@localhost. Click on the following link to confirm the registration, it is 48h valid from now.             
If the invitation was not intended, you can simply ignore this email.
$link"
          )
        );
        $this->lastError  = $request->getLastError();
    }
    return $this->success;
  }
  private function userExists($username, $email) {
      $sql = $this->user->getSQL();
      $res = $sql->select("User.name", "User.email")
          ->from("User")
          ->where(new Compare("User.name", $username), new Compare("User.email", $email))
          ->execute();
      $this->success = ($res !== FALSE);
      $this->lastError = $sql->getLastError();
      if ($this->success && !empty($res)) {
          $row = $res[0];
          if (strcasecmp($username, $row['name']) === 0) {
              return $this->createError("This username is already taken.");
          } else if (strcasecmp($username, $row['email']) === 0) {
              return $this->createError("This email address is already in use.");
          }
      }
      return $this->success;
    }
}