问没有指定模板id与签名的情况下为什么发送成功后有默认的签名？

$appid = '';
$appkey = '';
$random = rand(1000000,9999999);
$curTime = time();
$url = "https://yun.tim.qq.com/v5/tlssmssvr/sendsms?sdkappid=".$appid."&random=".$random;
$sig =  hash("sha256","appkey=".$appkey."&random=".$random."&time=".$curTime."&mobile=".$mobile, FALSE);
$param = array(
            "tel"=> [
                "nationcode"=> "86",
                "mobile"=> $mobile
            ], 
            "type"=> 0,
            "msg"=> $content, // “您的验证码是xxx”
            "sig"=> $sig,
            "time"=> $curTime, 
            "extend"=> "",
            "ext"=> "", 
);
$result = http_post($url,json_encode($param));