Yes, you should use classes to access your database.
A good link for further reading on that is to look up the Active Record Design Pattern and Object Relational Mapping
The benefit of this can be seen in the DRY Principle and the Law of Demeter question which was asked previously. By creating an abstract data access class, you keep the data access layer part of your application from leaking into your model/database entity classes AND you don't have to repeat yourself each time you have to make a new database model/entity.
Ok, so back to your question, so now, we build an abstract class to handle the CRUD for our database entities, so now, we only need to extend that abstract class and probably set some variables each time we add a new entity in the database. It makes the pain of writing SQL for your tables less annoying and much easier to handle by moving them all out to the abstract class as well as makes the data access layer of your application much manageable especially if you have a bazzillion models/database entities.
So whats the difference between making your abstract class and making a database model class then instantiating it with a database connection? Or the going straight to just using mysqli?
- With the abstract class, you only need to code the data access layer of your application ONCE
- Your code becomes DRY by eliminating the redundant database initializations
- Better maintainability, you only need to change one part of your code to change how you access the database!
- SQL Haters delight! They no longer need to directly touch SQL statements when dealing with ORMs
But as we are programmers, I would suggest not to rebuild this from scratch. No need to reinvent the wheel as they would say. There are already frameworks out there for PHP that handle ORM elegantly. You can look up popular MVC frameworks for PHP like Yii and Kohana.