提交给定次数的相同表单：为什么此代码不起作用？

So I am making a quiz in which the user gets confronted with a succession of questions which he answers through a form. The series of problems each contain a given number of questions, and the questions get asked one after the other when the user validates.

I am therefore trying to re-render the view with the form for each problem until they're all done. This is my action:

public function actionAnswer($id_serie)
{
    if ($id_serie != 0)                          //getting the serie's info
    {
        $serie = Serie::find()
            ->where(['id' => $id_serie])
            ->one();
        $problems = (new \yii\db\Query())        //getting the problems in the serie
            ->select('*')
            ->from('problems')
            ->where(['id_serie' => $id_serie])
            ->all();
        $prob_counter = $serie->nbr_of_problems; //counts the number of questions answered
        $id_serie = 0;
    }
    $model = new Answer;
    if ($model->load(Yii::$app->request->post()) && $model->validate())
    {
        $model->save();     // works just fine every time
        if (--$prob_counter <= 0)
        {
            return $this->redirect('index.php?r=student/entry');
        }
    }
    return $this->render('answer',
            ['model' => $model,
            'problems' => $problems,
            'serie' => $serie,
            'prob_counter' => $prob_counter,       //these last two are for debug
            'id_serie' => $id_serie]);
}

When this action gets executed the first time, $id_serie is never null or =0. Hence I am using this to query the db only once and set a counter to the total number of problems in the serie. (id est the number of time the user has to submit the form) If his answer is valid, I decrement my counter and if it falls under 0, there are no questions to answer anymore and the user gets redirected.

However, this counter never goes down to 0: it is set correctly, it is decremented only once, and then it never falls lower, no matter where I put the line. (inside or outside any loop) On the other hand the data from the form is properly inserted in the db each time.

What am I getting wrong?

As per your code, $prob_counter just stores the number of problems for each series. You need to change this to show the number of unanswered problems for the series. How you implement this will depend on your models and database but it should be something like:

$problems = (new \yii\db\Query())        //getting the problems in the serie
        ->select('*')
        ->from('problems')
        ->where(['id_serie' => $id_serie])
        ->andWhere('not exists (select id from answer where problemid = problems.id')
        ->all();

Also you should probably look at working with relational data and avoid using Query() in the above section.