解析数据时格式错误 JSON 响应

Malformed JSON response when parsing data

使用 Postman 查看 php 的这一部分是否按预期工作,但是 Postman returns 错误 Malformed JSON: Unexpected 'U' 并且数据库未更新,我无法理解这个错误对我来说没问题?

代码:

function placeVoteForCandidate()
{
    global $connect;

    $username = $_POST["username"];

    $query = "UPDATE User SET votesAttained=votesAttained+1 WHERE USER_NAME = $username";

    mysqli_query($connect, $query) or die (mysqli_error($connect));
    mysqli_close($connect);

    $message['success'] = 'Vote added';
    echo json_encode($message);
}

仅执行时

echo json_encode('Vote added');

应该没有问题。是否抛出任何异常导致此错误? 尝试将 SQL 语句更改为

$query = "UPDATE User SET votesAttained=votesAttained+1 WHERE USER_NAME = " . $username;