<?php
namespace Vnd\Extname\Domain;

class RequestValidator
    extends \TYPO3\CMS\Extbase\Validation\Validator\AbstractValidator
{
    public function isValid($req)
    {
        if ($req->foo == 5 && $req->bar == '') {
            //require $bar only when $foo is 5
            $this->result->forProperty('bar')->addError(
                new \TYPO3\CMS\Extbase\Validation\Error(
                    $this->translateErrorMessage(
                        'validator.notempty.empty',
                        'extbase'
                    ),
                    1221560910
                )
            );
        }
    }
}
?>