I've learned that OOP is all about data encapsulation, but what about passing data between classes that have nothing to do with each other (would the below example be worthy of using extends)?

class Dog {
    private $secretVar;

    public function getSecretVar() {
        $this->secretVar = 'psst... only for rainbow!';
        return $this->secretVar;
    }
}

class Rainbow {
    public function __construct(Dog $Dog) {
        print_r($Dog->getSecretVar());
    }
}

$Dog = new Dog();
$Rainbow = new Rainbow($Dog);

// ... classes that don't need the $secretVar

How would you encapsulate $secretVar for only classes Dog and Rainbow? As of now, anyone can call getSecretVar(), and I'm having a hard time allowing that to happen as it seems to defeat the whole point of encapsulation.