public function quoteIdentifier(string $identifier): string
    {
        // switch statement for DB type
        switch ($this->connect['type']) {
            case 'mysql':
                return '`' . str_replace('`', '``', $identifier) . '`';
            case 'postgresql':
            case 'sqlite':
            default:
                return '"' . str_replace('"', '\\' . '"', $identifier) . '"';
        }
    }