phpunit test exception if http answer code is 403

Question

I have my test code PHPUnit with selenium RC extension:

class LinkTest extends WebTestCase
{
 public function testShow()
 {
   $this->open('?r=link');
 }

Expression in open() return 403 HTTP code (Forbidden) and then PHPUnit call exception. This is not what i want.

I want to check is my url in open() is Forbidden or not. How can i do that with PHPUnit and SeleniumRC ?