/inc/MODEL folderThis folder holds the Data Model for the application.
The Data Model is the translation of the "real world" data into how this data is processed in the computer memory. In other words, it is also a PHP representation of the SQL DBMS data.
There will typically be one PHP Class for each DB table. Each object (instance of that class) roughly maps to a row in the table. Of course the Class may/should implement more integrity rules than the DB table alone.
There may also typically be one PHP Cacheing Object/Class (a Class with a single instance actually) for many tables in the DB. The Cache object will optimize accesses the DB by avoiding repeated queries on the same data.
It is important to note that the Data Model may also include data manipulation functions that do not take the form of a class.