在PHP程序里用以下语句调用jscode2session取openID，程序总算不出错了，可是结果为空，为什么？请教大家。谢谢！

$url = "https://open.weixin.qq.com/sns/jscode2session?appid={$app_id}&secret={$app_secret}&js_code={$login_code}&grant_type=authorization_code";

$result = geturl($url);

function geturl($url){

    $headerArray =array("Content-type:application/json;","Accept:application/json");

    $ch = curl_init();

    curl_setopt($ch, CURLOPT_URL, $url);

    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);

    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);

    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

    curl_setopt($ch,CURLOPT_HTTPHEADER,$headerArray);

    $output = curl_exec($ch);

    curl_close($ch);

    $output = json_decode($output,true);

    return $output;

}