/**
  * Authenticate user
  *
  * @return DataObjects_Users  returns users dataobject on success authentication
  *                            or null if user wasn't succesfully authenticated
  */
 function &authenticateUser()
 {
     $aCredentials = $this->_getCredentials();
     if (PEAR::isError($aCredentials)) {
         OA_Auth::displayError($aCredentials);
     }
     return $this->checkPassword($aCredentials['username'], $aCredentials['password']);
 }