code: public function validate($validator)
      {
          $value = $this->Value() + 0;
          
          if(is_int($value)) {
              return true;
          }
          
          $validator->validationError(
              $this->name,
              _t(
                  'Checkout.VALIDATION', '{value} is not a valid number, only whole numbers can be accepted for this field',
                  array('value' => $value)
              ),
              "validation"
          );
  
          return false;
      }
docs: PHP Validation
  
  @return boolean
  @throws Exception