curl 传入的URL如果是https请求返回结果bool(false)

public function getAccessToken(){
   $appid="wx109966137654df2d";
   $secret="e3776a8113cc6cc3f4108190097a4cb8";
   $url = 'https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid='.$appid.'&secret='.$secret;
   $ch = curl_init();
   curl_setopt($ch,CURLOPT_URL,$url);
   curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
   $res = curl_exec($ch);
   curl_close($ch);
   if (curl_errno($ch)){
       var_dump(curl_errno($ch));
   }
   $arr = json_decode($res,true);
   var_dump($arr);

   var_dump($res);
}