//第一次请求登录接口时保存到sessionid中

success: function (res) {
     wx.hideLoading();
     wx.removeStorageSync('sessionid');//每次登录时清楚缓存
     if (res.data.code == "0000") {
      if (res.data.data.roleList[0].value =='student'){
       wx.showToast({
        title: '登录成功',
        icon: 'success',
        duration: 1000
       })
       wx.setStorageSync('sessionid', res.header['Set-Cookie']); //保存Cookie到Storage
        
      }
     } else {
      wx.showToast({
       title: '登录失败',
       icon: 'none',
       duration: 2000
      })
     }
    },

wx.request({   url: 'https://xxxx/examResult/selectWxListStudentStudyStatistic',
   method: 'post',
   header: {
    'content-type': 'application/x-www-form-urlencoded',//解决请求不到数据
    'cookie': wx.getStorageSync("sessionid")//读取sessionid,当作cookie传入后台将PHPSESSID做session_id使用
   },
   success: function (res) {
     wx.hideLoading();//去掉loading
   },
   fail: function (e) {
    wx.showToast({
     title: '服务器出现错误',
     icon: 'none',
     duration: 2000
    })
   }
  })