60：SSL证书：证书chainbool中的自签名证书

我尝试使用我的正确APP_ID，APP_SECRET等发送curl请求到

I try to send curl request with my correct APP_ID, APP_SECRET etc. to the

  https://oauth.vk.com/access_token?client_id=APP_ID&client_secret=APP_SECRET&code=7a6fa4dff77a228eeda56603b8f53806c883f011c40b72630bb50df056f6479e52a&redirect_uri=REDIRECT_URI 

需要从它获取access_token，但得到一个FALSE和 curl_error（）打印下一条消息否则：

I need to get access_token from it, but get a FALSE and curl_error() print next message otherwise:

60: SSL certificate problem: self signed certificate in certificate chain

我的代码是：

    // create curl resource
    $ch = curl_init();

    // set url
    curl_setopt($ch, CURLOPT_URL, $url);
    //return the transfer as a string
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

    // $output contains the output string
    $output = curl_exec($ch);
    if ( ! $output) {
        print curl_errno($ch) .': '. curl_error($ch);
    }

    // close curl resource to free up system resources
    curl_close($ch);

    return $output;

当我手动移动到上面的链接，我得到access_token很好。为什么它不工作curl？

When I move manually to the link above, I get access_token well. Why it doesn't work with curl? Help, please.