MySQL and MySQLi (MySQL improved) are both PHP extensions used to connect to and interact with MySQL databases, but there are some key differences between the two:

1. API:  MySQL uses the original MySQL API, while MySQLi uses the improved MySQLi API.
2. Object-oriented vs procedural programming: MySQLi supports both object-oriented and procedural programming, while MySQL only supports procedural programming.
3. Prepared statements and bound parameters: MySQLi supports prepared statements and bound parameters, which allows for more secure database queries, while MySQL does not.
4. Transactions: MySQLi supports transactions, while MySQL does not.
5. Performance: MySQLi is generally faster than MySQL due to its improved API and the use of prepared statements.

Overall, MySQLi is considered to be the better choice for new projects and development, as it offers more functionality and security. However, if you are working with an existing application that uses MySQL, it may be more practical to continue using MySQL.