Reinitializing variable in php to NULL - php

Question

I'm writing a simple gamePage. My Controller method looks like this:
$model = new Game143();
if ($model->load(Yii::$app->request->post()) && $model->validate()) {
$model->verifyAnswers();
return $this->render('Game143-confirm', ['model' => $model]);
} else {
$model->initGame(1, 2);
return $this->render('Game143', ['model' => $model]);
}
The else is the important bit. The initGame method is passed the starting level and the max number of levels, so I'm iterating 2 times with random digits and everything works fine.
In my init method I set the value:
public function initGame($level, $numEx) {
$this->maxLevel = $numEx; // normally 10
$this->thisLevel = $level; //normally 1
I also did some output. This works fine and my variable is saved as 2.
In the if method of the controller my variable is suddenly set to NULL?!
Because of this even my results are NULL, because my iteration is <=maxLevel. Anyone know why this happens?
In verifyAnswers(); my given and initialized maxLevel is NULL.
Iterating not possible:
for(; $this->thisLevel <= $this->maxLevel; $this->thisLevel++) {
If i hardcode the maxLevel everything works fine also.
In the view (Working with yii-2):
for($model->thisLevel = 1; $model->thisLevel <= $model->maxLevel; $model->thisLevel++) {
$out .= "\n";
$out .= Html::activeHiddenInput($model,"digit1[$model->thisLevel]");
$out .= "\n";
$out .= Html::activeHiddenInput($model,"digit2[$model->thisLevel]");
$out .= "\n";
$out .= "\n";
$out .= "<h3>Level $model->thisLevel</h3>\n";
$out .= "<h3>max $model->maxLevel</h3>\n";
$out .= "\n";
$out .= "<div class=\"row aufgabenFeld\">";
$out .= "<div align=\"right\" class=\"col-lg-4 col-md-4 col-xs-4 wallLabel \">";
$out .= $model->digit1[$model->thisLevel];
$out .= "</div>";
$out.="<div id='myField__<?php echo $model->thisLevel; ?>' class=\"col-lg-2 col-md-2 col-xs-2 wall\">\n";
$out .= $form->field($model, "proposal[$model->thisLevel]")->label(false);
$out .= "</div>";
$out .= "<div align=\"left\" class=\"col-lg-4 col-md-4 col-xs-4 wallLabel \">"; //id test xD
$out .= $model->digit2[$model->thisLevel];
$out .= "</div>";
$out .= "</div>";
$out .= "<div><br></div>";
In the 'submitView':
foreach ($model->proposal as $key=>$value){
$out .= "<li><h4>Aufgabe $key</h4>";
$out .= $model->digit1[$key];
$out .= " ";
$out .= $model->proposal[$key];
$out .= " ";
$out .= $model->digit2[$key];
$out .= $model->maxLevel[$key];
$out .= "</li>";
}