Monetary Values in PHP – How to Handle Monetary Values in PHP and MySQL?

There do seem to be PHP wrappers for OpenCV, such as OpenCV For PHP.

Now, for the recognition part, I think that the best way to go is to use Artificial Neural Networks. ANN's can be trained to identify people within images and are more suited because they can handle relatively inconsistent inputs and produce relatively stable results. There does seem to be ANN implementations in PHP, such as FANN. The implementation of an ANN's should also be quite fast (once trained, that is) and will remove the need for a database, unless you will be using it for other things and/or persist the neural network itself.

This would mean that you can write your solution entirely in PHP.

That being said, my experience with wrappers is that you do not always get all the goodies provided by the original library, so with that in mind, you could also opt to have PHP handle your front end and then, use C++ as back end, which should allow you to exploit all the power of OpenCV (which could come in handy for future add-ons).

As for communication between the two layers, I think using standard SOAP web service communication (for PHP and C/C++) will provide you with a robust solution.

The purpose of the repository is to provide an adapter around the database. It "knows about the database" so the rest of your code doesn't have to.

So it seems quite reasonable to me that if you change your database schema, you'll have to change (or altogether replace) your repositories. The point is that you only have to replace the repositories, because the implementation is hidden from the application by the interface.

If you have lots of aggregates and a repository for each of them, so replacing them wholesale is hard, you might be able to get help from an ORM, depending on the technologies you're using. A generic Repository<T> seems to be pretty common in C#, for example.

Of course, even if you're using an ORM, there still has to be some code, somewhere, which knows about the database schema. But this way it's usually tucked away in an XML mapping file (which of course has its own advantages and disadvantages).

Edit in response to your comments:

I don't really see what you're trying to achieve. In your design, it looks like the 'Hydration' objects take on the responsibility of communicating with the database and returning a domain object. So if you change your database schema, you still have to change all the Hydration objects. Your proposal does not solve your perceived problem: there has to be some code somewhere which knows about the database schema.

But your design is so much more complicated: there's an extra layer (the Repository itself) which basically does nothing.

public class FooRepository
{
    private IFooHydration _fooHydration;

    public FooRepository(IFooHydration fooHydration)
    {
        _fooHydration = fooHydration;
    }

    public Foo Get(int id)
    {
        return _fooHydration.Get(id);
    }

    public void Save(Foo entity)
    {
        _fooHydration.Save(entity);
    }
}

It's much simpler to use an IRepository, and write different implementations of that for your different persistence methods. In short, the Repository is what you plug-in when you change databases.