There is no direct train from Zhangjiajie to Fenghuang. You can take the direct bus from the passenger transportation center at Zhangjiajie city center to Fenghuang. The bus departs at 08:00 and 14:30 and it takes about 4 hours. I am not sure about the taxi fare but it is really exoensive. I suggest you take the tourist bus from Fenghuang to Changsha West bus station and then take taxi to Changsha airport. Or take bus to Jishou and then take train from Jishou to Changsha.