code: public function authenticate($identity, $credential)
      {
          $adapter = $this->auth->getAdapter();
          $adapter->setIdentity($identity);
          $adapter->setCredential($credential);
  
          return $this->auth->authenticate();
      }
docs: Authenticates user.
  
  @param string $identity   User identifier (username, email, etc)
  @param string $credential User password
  
  @return Zend\Authentication\Result
  
  @throws Zend\Authentication\Exception\RuntimeException