PDO赢得'；我没有更新数据库中的数据，为什么

我正在尝试使用PDO更新我的数据库，但它无法工作。我可以选择没有任何问题的数据，但更新是不起作用的。我没有收到错误消息，但在执行查询后，数据库中没有任何变化。

这是我的代码，这个方法是一个类的一部分，所以定义了pdo，但我不会把整个代码粘贴到这里，只是重要的部分：

public function modify($q, $data){
        try{
            $stmt = $this->pdo->prepare($q);
            $stmt->execute($data);
            return true;
        }
        catch(PDOException $e){
            $this->error = $e->getMessage();
            return false;
        }
    }

和：

$question = new Question(base64_decode($_POST['value']), $_POST['question'], $_POST['code'], $_POST['number'], $_POST['answer1'],
    $_POST['answer2'], $_POST['answer3'], $_POST['answer4'], $_POST['correct_answer']);
$array = [
    ':qid' => $question->qid,
    ':question' => $question->question,
    ':code' => $question->code,
    ':number' => $question->number ,
    ':answer1' => $question->answer1,
    ':answer2' => $question->answer2,
    ':answer3' => $question->answer3,
    ':answer4' => $question->answer4,
    ':correct_answer' => $question->correct_answer#9
];
$q = 'UPDATE questions SET question = :question AND code = :code AND number = :number AND
        answer_1 = :answer1 AND answer_2 = :answer2 AND answer_3 = :answer3 AND answer_4 = :answer4
        AND correct_answer = :correct_answer WHERE question_id = :qid';
$db->query('SET NAMES UTF8');
$db->modify($q, $array);

谢谢！