Declare class property at runtime, in Yii2

I have a class that extends from Yii2's Model and I need to declare a class public property in the constructor, but I'm hitting a problem.

When I call

class Test extends \yii\base\Model {
    public function __constructor() {
        $test = "test_prop";
        $this->{$test} = null; // create $this->test_prop;
    }
}

Yii tries to call, from what I understand, the getter method of this property, which of course doesn't exist, so I hit this exception.

Also, when I actually do $this->{$test} = null;, this method gets called.

My question is: Is there a way to declare a class public property in another way? Maybe some Reflexion trick?