code: public function authenticate(Zend_Auth_Adapter_Interface $adapter)
      {
          // Authenticates against the supplied adapter.
          $result = $adapter->authenticate();
  
          /*
           * ZF-7546 - prevent multiple succesive calls from storing inconsistent
           * results.
           *
           * Ensure storage has clean state.
           */
          if ($this->hasIdentity()) {
              $this->clearIdentity();
          }
  
          if ($result->isValid()) {
              $this->getStorage()
                  ->write($adapter->getResultRowObject());
          }
  
          return $result;
      }
docs: Authenticates against the supplied adapter.
  
  @param Zend_Auth_Adapter_Interface $adapter The adapter to use
  @return Zend_Auth_Result
  @see http://framework.zend.com/manual/1.12/en/zend.auth.introduction.html#zend.auth.introduction.adapters Zend_Auth_Adapter_Interface
  @see http://framework.zend.com/manual/1.12/en/zend.auth.introduction.html#zend.auth.introduction.results Zend_Auth_Result