The mysql_* functions in PHP are a set of functions that provide access to MySQL databases. These functions have been deprecated in PHP version 5.5.0 and have been removed in PHP version 7.0.0.

The mysql_* functions are no longer recommended for several reasons:

1. They are vulnerable to SQL injection attacks: The mysql_* functions do not support prepared statements or parameterized queries, which makes them vulnerable to SQL injection attacks. SQL injection attacks allow an attacker to execute arbitrary SQL commands by injecting malicious code into a vulnerable query. This can compromise the security of your database and your application.
2. They are not portable: The mysql_* functions are specific to MySQL and do not work with other database systems. If you want to switch to a different database system, you will have to rewrite all of your database code.
3. They are inefficient: The mysql_* functions use a proprietary client library to communicate with the MySQL server. This library is not optimized for performance, and can be slower and more resource-intensive than other options.
4. They do not support new features: The mysql_* functions do not support new features that have been added to MySQL in recent versions, such as prepared statements and multi-query support.

For these reasons, it is recommended to use a modern database abstraction library, such as PDO or MySQLi, instead of the mysql_* functions. These libraries provide a uniform interface for accessing different types of databases, and support prepared statements and parameterized queries to help prevent SQL injection attacks. They are also more efficient and portable, and support new features in MySQL.

To migrate from the mysql_* functions to PDO or MySQLi, you will need to rewrite your database code to use the new functions. This can be a time-consuming process, but it is worth it in the long run to improve the security, and performance.