How to understand the following code?

Question

Yii

There is a method

public function myRule($attribute, $params)
  {
    if (!in_array($this->$attribute, ['USA', 'world'])){
      $this->addError($attribute, 'Страна должна быть либо "USA" или "Indonesia".');
    }
  }

What does $this->$attribute refer to in this code?
To check the argument of a method, it was necessary like if (!in_array($attribute, ['USA', 'world']))
this. Why is this needed here?

Answer 1

What does $this->$attribute refer to in this code?

public function rules() {
  return [
    ['country', 'myRule']
 ];
}

public function myRule($attribute, $params)
  {
    //$attribute == 'country' - поле в модели
    //$this->$attribute == $this->country - значение поля в модели
    if (!in_array($this->$attribute, ['USA', 'world'])){
      $this->addError($attribute, 'Страна должна быть либо "USA" или "Indonesia".');
    }
  }

Answer 2

check out the implementation of OOP in php