zend_class_entry.create_object

When registering a new class, what are the consequences of setting zend_class_entry.create_object = NULL? INIT_CLASS_ENTRY() defaults it to NULL, but the tutorial I'm looking at sets it to the address of a function immediately afterwards. My concern is that setting this allows the engine to call a constructor-like-function behind my back. I'd rather handle such things while handling calls to new(). Is that possible? Is it even a good idea? The tutorial code provides a function for zend_class_entry.create_object that returns a populated zend_object_value. Presumably that's a useful thing to do, and if I don't have a function there I would need to find some other means of supplying the zend_object_value to the engine. Can I do that in a new() handler? How?