PDO :: FETCH_OBJ没有返回结果？

Question

我有一个具有下表的数据库

对象

-------------------------------------------------------
id | id_group | title | description | lat | lng | icon
-------------------------------------------------------

我想创建一个json作为查询的答案，该查询提供纬度和经度以及半径。

<?php
header("Access-Control-Allow-Origin: *");
header("Content-Type: application/json; charset=UTF-8");
header("Access-Control-Allow-Methods: POST");
header("Access-Control-Max-Age: 3600");
header("Access-Control-Allow-Headers: Content-Type, Access-Control-Allow- 
Headers, Authorization, X-Requested-With");
// get database connection
include_once '../config/database.php';
$database = new Database();
$db = $database->getConnection();
// get posted data
$latitude = $_GET['lat'];
$longtitude = $_GET['lng'];
$radius = $_GET['radius'];

if(
!empty($latitude) &&
!empty($longtitude) &&
!empty($radius)
){

    $qry = "SELECT *,
    (
        6371 * acos
        (
            cos(
                radians( :lat )
            )
            * cos(
                radians( lat)
            ) * cos(
                radians( lng) - radians( :lng )
            ) + sin(
                radians( :lat )
            ) * sin(
                radians( lat)
            )
            )
            ) AS distance
            FROM objects
            HAVING distance <= :radius
            ORDER BY distance ASC;";

    $stmt = $db->prepare($qry);

    $stmt->bindParam(":lat", $latitude);
    $stmt->bindParam(":lng", $longtitude);
    $stmt->bindParam(":radius", $radius);

    if($stmt->execute()){
        while ($result = $stmt->fetch(PDO::FETCH_OBJ)) {
            echo json_encode(array("type" => "FeatureCollection",
                "features" => array(
                    "type" => "Feature",
                    "geometry" => array("type" => "Point",
                        "coordinates" => [$latitude, $longtitude]),
                    "properties" => array(
                        "ID" => $result->id,
                        "icon" => $result->icon,
                        "tags" => "",
                        "title" => $result->title,
                        "description" => $result->description))));
        }

    }

    else{

        http_response_code(503);

        echo json_encode(array("message" => "Error!"));
    }
}
else{

http_response_code(400);

echo json_encode(array("message" => "Data imcomplete"));
}
?>

执行后的while循环不起作用（即使执行很好），fetchColumn（）在这里也不起作用，即使我有一个类似的代码也可以在相同的输入下很好地工作。