JavaScript-PHP JSON_encode无法正常工作

我想从this获取this JSON输出.不幸的是json_encode()函数不会将数组编码为该格式.一切都没有回报.这是我的代码.

$output = array(
    'responseData' => array(),
    'responseDetails' => null,
    'responseStatus' => 200
);

$x = 0;
while ($row = MysqLi_fetch_assoc($result)) {
    foreach ($row as $k => $v) {
        $output['responseData']['result'][$x][$k] = $v;
    }
    $x++;
}

print_r($output);
header('Content-Type: application/json');
echo json_encode($output , JSON_FORCE_OBJECT);

我找不到原因.有人请帮助我找到解决方案.

编辑：对不起.这是输出-

预期的JSON输出-

{
"responseData": {
    "results": [{
        "qid": 1,
        "qtitle": "When do we finish this project ?",
        "qimage_url": "http://www.wearesliit.com/example.png",
        "user": "samith",
        "date": "2016-01-01T02:15:12.356Z",
        "type": 1,
        "category": 5,
        "tags": ["common_senese", "truth", "bazsa_awsanna"],
        "note": "Sample quetion"
    }, {}, {}]
},
"responseDetails": null,
"responseStatus": 200 }

我根本没有得到任何JSON输出.但是这是数组的print_r结果.

 Array(
[responseData] => Array
    (
        [result] => Array
            (
                [0] => Array
                    (
                        [question_ID] => 1
                        [question_Title] => Which shape does not belong with the other three shapes?
                        [question_Image_URL] => http://www.wearesliit.com/images/quiz/questions/1.jpg
                        [quetion_Note] => Easy IQ question.
                        [category_ID] => 7
                        [username] => samith
                        [added] => 2017-01-29 21:50:52
                    )

                [1] => Array
                    (
                        [question_ID] => 2
                        [question_Title] => Tim earns $10 per hour at his job.  When he gets paid on Friday, he is paid for 40 hours of work.  He then goes out and spends 10% of his earnings on entertainment that weekend.  How much money is he left with on Monday?
                        [question_Image_URL] => 
                        [quetion_Note] => Easy IQ question.
                        [category_ID] => 7
                        [username] => samith
                        [added] => 2017-01-29 21:50:52
                    )
            )

    )

[responseDetails] => 
[responseStatus] => 200 )

解决方法:

感谢@awiebe,我找到了确切的错误.它的

Malformed UTF-8 characters, possibly incorrectly encoded

谢谢大家,我从另一个问题中找到了解决方案.
‘Malformed UTF-8 characters, possibly incorrectly encoded’ in Laravel