eZ\Publish\Core\FieldType\ISBN\Type::validateFieldSettings PHP Method

    public function validateFieldSettings($fieldSettings)
    {
        $validationErrors = array();
        foreach ($fieldSettings as $name => $value) {
            if (!isset($this->settingsSchema[$name])) {
                $validationErrors[] = new ValidationError("Setting '%setting%' is unknown", null, array('%setting%' => $name), "[{$name}]");
                continue;
            }
            switch ($name) {
                case 'isISBN13':
                    if (!is_bool($value)) {
                        $validationErrors[] = new ValidationError("Setting '%setting%' value must be of boolean type", null, array('%setting%' => $name), "[{$name}]");
                    }
                    break;
            }
        }
        return $validationErrors;
    }