使用mysqli_fetch_assoc（）构建一个id数组

I'm trying to use mysqli to get an array of 'questions' which 'users' have answered. I have a questions_users table to achieve this and I'm using the following code to find a user and then get all the questions they've answered, and build an array of their id's so I can exclude these questions from being picked as a new question to answer.

$qusAnswered = array();
$DoWeHaveUsersql = "SELECT * FROM users WHERE `checkid`=".$checkid;
$DoWeHaveUserresult = $conn->query($DoWeHaveUsersql);


if ($DoWeHaveUserresult->num_rows > 0) {//we've got a user
    $row = mysqli_fetch_assoc($DoWeHaveUserresult);

    $userId = $row['id'];
    $checkQuestionSql = "SELECT * FROM questions_users WHERE `user_id`=".$userId;//get all the questions_users records

    $checkQuresult = mysqli_query($conn, $checkQuestionSql);

    if (mysqli_num_rows($checkQuresult) > 0) {
        // make an array of all the id's of questions answered
        while($Qu = mysqli_fetch_assoc($checkQuresult)) {
            $qusAnswered[] = $Qu['question_id'];
        }
    } 
    $newQuestionSql = "SELECT * FROM questions WHERE id NOT IN (".implode(',', $qusAnswered).") LIMIT 1";
}

The problem is when I'm building the $qusAnswered array. From what's in the database it should contain only the value 3, because thats the id of the question they've answered. For some reason $qusAnswered actually looks like

array('1','2');

I'm not sure what I'm doing wrong and if anyone can point out the obvious for me I'd appreciate it.

the SQL which is in the var $newQuestionSql looks like:

SELECT * FROM questions WHERE id NOT IN (1,2) LIMIT 1

it should be:

SELECT * FROM questions WHERE id NOT IN (3) LIMIT 1

Well my join-fu is not as good as I thought, but here's how to do this in one query, and without opening yourself to SQL injection attacks:

$query = "SELECT * FROM questions WHERE question_id NOT IN (
    SELECT question_id FROM questions_users WHERE user_id=?
);"
$userid = $_GET["user_id"];
$stmt = $conn->prepare($query);
$stmt->bind_param("i", $userid);
$stmt->execute();
$result = $stmt->get_result();
$rows = $result->fetch_all(MYSQLI_ASSOC);
foreach ($rows as $row) {
    //do stuff with $row
}

I would like to suggest you just simple one query.

Might be it will solve your problem

SELECT * from questions where id IN (SELECT GROUP_CONCAT(question_id) FROM `questions_users` WHERE `user_id`=1)