Well, Steve, I called it main_class just for an example purpose. The
fact is this is supposed to be a class that holds several class in
only one, so I could call:

$main_class->database->method($args);
$main_class->html->make_form($args);

Understood? I think this is much better than having several variables
for different classes, as it groups the whole functionality of all
classes into only one variable. If you find there is a way to do this
better, please, do tell me.