Trying to get property of non-object

[hyyyp]

`public function success_validate($challenge, $validate, $seccode,$param, $json_format=1) {
if (!$this->check_validate($challenge, $validate)) {
return 0;
}
$query = array(
"seccode" => $seccode,
"timestamp"=>time(),
"challenge"=>$challenge,
"captchaid"=>$this->captcha_id,
"json_format"=>$json_format,
"sdk" => self::GT_SDK_VERSION
);
$query = array_merge($query,$param);
$url = "http://api.geetest.com/validate.php";
$codevalidate = $this->post_request($url, $query); //当验证码失效后 再次提交 返回false

    $obj = json_decode($codevalidate);

#建议判断 $obj === false 选择返回!
if ($obj->{'seccode'} == md5($seccode)) { # 报错
return 1;
} else {
return 0;
}
}`