即使插入了数据，MySqli语句execute也返回false

这是我的简单瘦应用程序。下面的这个函数处理插入。

public function createSelfie($user_id, $image, $latitude, $longitude) {
        $stmt = $this->conn->prepare("INSERT INTO user_selfie(image, latitude, longitude, user_id) VALUES(?,?,?,?)");
        $stmt->bind_param("sssi", $image, $latitude, $longitude, $user_id);
        $result = $stmt->execute();
        $stmt->close();
        if ($result) {
            // we were successful in storing the selfie now we need to manipulate the 'user_egg' table values
            return SELFIE_CREATED_SUCCESSFULLY;
        }
        else {
            // selfie failed to create
            return NULL;
        }
    }

为此，路线如下：

$app->post('/selfie/create', function() use ($app) {
            verifyRequiredParams(array('image', 'latitude', 'longitude', 'user_id'));
            // reading post params
            $image = $app->request()->post('image');
            $latitude = $app->request()->post('latitude');
            $longitude = $app->request()->post('longitude');
            $user_id = $app->request()->post('user_id');
            $response = array();
             $db = new DbHandler();
             $res = $db->createSelfie($user_id, $image, $latitude, $longitude);
             if ($res == SELFIE_CREATED_SUCCESSFULLY) {
                $response['error'] = false;
                $response["message"] = "Your selfie created successfully";
                echoRespnse(201, $response);
             }
             else {
                $response['error'] = true;
                $response["message"] = "There was an error storing your selfie try again";
                echoRespnse(200, $response);
             }
        });

现在，有时它会正确响应，有时则为空。但每次执行语句时，我都会看到行被插入。然后我很困惑为什么函数返回false。原因可能是什么？如果我们不知道原因，那么使用affected_rows而不是result是一种好的做法吗？。附带说明一下，我确实有一个echoResponse方法，它对结果进行json编码，并将响应作为json发送。