I'm coding a WP plugin using PHP OOP. I'm passing some classes to my main class, so that I will be able to access variables and methods from my main class to other classes. In fact I'm able to access methods, but not variables. For example we have Main and Secondary classes:

class Main {
    public $secondary;

    public function __construct($secondary) {
        $this->secondary= $secondary;
    }

    public function methodFromSecondary() {
        echo $this->secondary->secondaryMethod();
    }

    public function variableFromSecondary() {
        echo $this->secondary->secondaryVariable;
    }
}

class Secondary {
   public $secondaryVariable;

   public function __construct() {
       $this->secondaryVariable = 'some variable string';
   }

   public function secondaryMethod() {
       return 'some mothod string';
   }
}

$secondary = new Secondary();
$main = new Main($secondary);

If I call $main->methodFromSecondary() it outputs 'some mothod string' and if I call $main->variableFromSecondary(), WP outputs that site is experiencing technical issues. So how can I access a variable to output 'some variable string' as I access method?