The MySQL extension:
- Is not under active development
- Is officially deprecated as of PHP 5.5 (released June 2013).
- Has been removed entirely as of PHP 7.0 (released December 2015)
- Lacks an OO interface
- Doesn’t support:
- Non-blocking, asynchronous queries
- Prepared statements or parameterized queries
- Stored procedures
- Multiple Statements
- The “new” password authentication method (on by default in MySQL 5.6; required in 5.7)
- All of the functionality in MySQL 5.1
Since it is deprecated, using it makes your code less future proof.
Lack of support for prepared statements is particularly important as they provide a clearer, less error prone method of escaping and quoting external data than manually escaping it with a separate function call.