********************************************************************************************************
    CREATE TABLE IF NOT EXISTS `wxtoken` (
  `id` int(11) NOT NULL,
  `appid` varchar(100) NOT NULL,
  `appsecret` varchar(100) NOT NULL,
  `access_token` varchar(1024) DEFAULT NULL,
  `AccessExpires` int(11) DEFAULT NULL,
  `TIMESTAMP` timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
    ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;


INSERT INTO `wxtoken` (`id`, `appid`, `appsecret`, `access_token`, `AccessExpires`, `TIMESTAMP`) VALUES
(1, '你的appid', '你的appsecret', '****', 1543244037, '2018-11-26 12:53:57');


ALTER TABLE `wxtoken`
  ADD PRIMARY KEY (`id`);


ALTER TABLE `wxtoken`
  MODIFY `id` int(11) NOT NULL AUTO_INCREMENT,AUTO_INCREMENT=2;


    以上SQL语句,请在数据库中执行.
********************************************************************************************************


    public function index()
    {
        $access_token = $this->getToken();
        echo $access_token;
    }



    function getToken(){
        return $this->checkAccessToken("你的公众号appid","你的公众号appsecret");
    }



    function checkAccessToken($appid,$appsecret){
        $condition = array('appid'=>$appid,'appsecret'=>$appsecret);
        $access_token_set=DB('wxtoken')->where($condition)->find();//获取数据

        if($access_token_set){
            //检查是否超时，超时了重新获取
            if($access_token_set['AccessExpires']>time()){
                //未超时，直接返回access_token
                return $access_token_set['access_token'];
            }else{
                //已超时，重新获取
                $url_get='https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid='.$appid.'&secret='.$appsecret;
                $json= $this->https_request($url_get);
                var_dump($json);
                $access_token=$json['access_token'];
                $AccessExpires=time()+intval($json['expires_in']);
                $data['access_token']=$access_token;
                $data['AccessExpires']=$AccessExpires;
                $result = DB('wxtoken')->where($condition)->update($data);//更新数据
                if($result){
                    return $access_token;
                }else{
                    return $access_token;
                }
            }
        }else{
          	echo "appid或appsecret不正确";
            return false;
        }
    }


    function https_request ($url){
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        $out = curl_exec($ch);
        curl_close($ch);
        return  json_decode($out,true);
    }