Covariance testing

<?php
class Base
{
    protected $foo = null;
    final public function __construct(Granny $init = null)
    {
        return $this->init($init);
    }
    protected function init(Granny $dependency = null)
    {
        throw new Exception("Don't run me from Base, foo!");
    }
}
class Granny
{
    protected $name = null;
    protected $age = null;
    public function __construct(array $vals = null)
    {
        foreach($vals as $name => $val)
        {
            $name = 'set'.ucfirst($name);
            if (method_exists($this, $name))
            {
                $this->{$name}($val);
            }
        }
        return $this;
    }
    //basic gettter && setters
    public function getAge()
    {
        return $this->age;
    }
    public function setAge($age = null)
    {
        $this->age = $age === null ? null : (int) $age;
        return $this;
    }
}

class Dad extends Granny
{
    //Dad's secret
    private $likesMom = null;
    public function getLikesMom()
    {
        return $this->likesMom;
    }
    public function setLikesMom($bool = null)
    {
        $this->likesMom = $bool === null ? null : !!$bool;
        return $this;
    }
}

class Ball extends Base
{
    protected function init(Dad $dependency = null)
    {
        $this->foo = $dependency;
        return $this;
    }
}

var_dump(new Ball(new Dad()));