SimpleTest testing composite objects.

Hi

I'm trying to start using Test Driven Development for my projects. I
have red few blogs and look on Google but have not found anything useful
.. My question is how to write a tests to test complex composite objects. Ex:

class A{
getDB($name){
$this->db = DB::create_db($name);
}
}

Where DB is factory class returning mysqli instantiated class.

Anyone can tell me or point me in the right direction how to write a
tests for this kind of objects.

Ralph