Overriding Zend_Form isValid()
public function isValid($data) {
// conditional requirement of fields
$isValid = parent::isValid($data);
if ($isValid) {
// additional validation
}
return $isValid;
}
http://stackoverflow.com/questions/3197216/drawbacks-to-overriding-the-isvalid-function-on-zend-form
http://zend-framework-community.634137.n4.nabble.com/Overriding-Zend-Form-isValid-td1018622.html