The reasons are as follows:
The MySQL extension:
- Does not come 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)
- It lacks an OO interface
- Also does not 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, I would suggest not using it, as it would make your code less future proof.