The base DAO provides methods to the VehicleDAO class (and any other classes that extend it)
that call database specific PHP methods to access the database. The base DAO could be used on
it's own to provide data access on a PHP page, but in keeping with the MVC concepts, we use it
as an abstract class whose methods are implemented by objects that know about specific entities
in the database.
The Extended Data Access Object
The VehicleDAO class implements the functionality of the base DAO and transfers data between
VehicleVO's and the database. The constructor calls the base DAO's constructor which establishes
the connection to the database.