private function canonicalizeFieldValue($fieldname, $fieldvalue)
    {
        switch ($fieldname) {
            case 'email':
                return strtolower(trim($fieldvalue));

            case 'username':
                return strtolower(preg_replace('/[^a-zA-Z0-9_\\-]/', '', $fieldvalue));

            default:
                return trim($fieldvalue);
        }
    }