class class_one {

    protected $value;
    private $obj_two;

    public function __construct() {
        $this->obj_two = new class_two;
    }

    public function firstFunction() {

        $this->obj_two->obj_two_function();

        echo $this->value; // returns 'New Value' like set in the class two

    }

}

class class_two extends one {
    public function obj_two_function() {
        "class_one"->value = 'New Value';
    }
}