public function execute($sql, $params = null)
    {
        $stmt = $this->_pdo->prepare($sql);

        if (is_array($params)) {
            foreach ($params as $token => $value) {
                $stmt->bindValue($token, $value);
            }
        }

        try {
            $stmt->execute();
        } catch (PDOException $exception) {
            $this->_database->error('execute', $exception);
        }

        return $stmt;
    }