class SomethingInputForm extends BaseForm
{
    public function configure()
    {
        // ...
        $this->validatorSchema->setPostValidator(
            new sfValidatorCallback(array('callback' => array($this, 'myCallbackFunc')))
        );
    }

    public function myCallbackFunc($validator, $values) {
        // 何かバリデーション
        if (!($values['input1'] == $values['input2'] == $values['input3'])) {
            $error = new sfValidatorError($validator, 'error message ...');
            throw new sfValidatorErrorSchema($validator, array('input1' => $error));
        }

        return $values;
    }
}