A very common pattern I see (I'm picking on Zend Framework, only because I was dealing with it at the moment of this question), is something like this:

class My_Form extends Zend_Form {
     public function init() {
          $this->addElement();
     }
}

Zend_Form is not an abstract class, but is perfectly usable on its own. This seems to be "recommended" as place to "encapsulate" your forms into a nice class.

Does this violate the Liskov Substitution Principle? Each subclass of Zend_Form will have a wildy different behavior than the base class. Would it be better to use composition for this, or am I totally misunderstanding this principle?