public function configure()
{
	$this->validatorSchema->setPostValidator(
    	new sfValidatorCallback(array(
        	'callback' => array($this, 'myCallback')
    	))
 	);
}

public function myCallback($validator, $values)
{
	if ($values["reject"] == 1) 
    	{
    		if ($values["reject_reason"] == "")
	    	{
    			throw new sfValidatorError($validator, 'Please enter a reason');	
	    	}
	}
	return $values;
}