Distinguishing between libraries? (Directory structure)

So, here's a quick extract of the directory structure on a web application that I am working on currently:

/app
  /libraries
    User_Input.php
    Database.php
    InvoiceHandler.php

Do you think that the InvoiceHandler is a bit odd in this collection? I sure do.

Until recently, I have ignored the need to distinguish between de-facto libraries that actually are the middle-layer between my application and other services (such as a Persistent storage) and the cl开发者_StackOverflow中文版asses of code that I have written myself that contains custom logic for my application (such as the Invoice_Handler in this case).

But as my application grows, this creates more and more of a mess. I haven't seen any good examples on this as of today and I have not enough experience to come to a conclusion by myself:

What does conventions and common sense say on separation of these two definite and so clearly different sort of classes?

You can use the following structure:

libs
    Common(Core)
        Db
          Connection(Database).php
    Input
         Validator
         Filter
    Handler(Controller)
        Invoice.php
    Model 

This question is a bit vague, and the example filenames shown don't really explain much about them. I will however say that you need to group your classes together into a structure that makes sense.

/app
  /library
    /My
      /Controller
        AbstractController.php
      /Filter
        CamelCaseToUnderscore.php
        UnderscoreToCamelCase.php
      /Validator
        Date.php
        Url.php

The classes I've used are vastly different (at least I think they are) from yours, but you can clearly see they are grouped into a logical structure.

I ended up separating my libraries depending on what level of abstraction they contained. In search of a better name of high-level abstraction, I used the category name services for these and retained with libraries for low-level abstraction classes.

Ie. :

/common
    /libraries (low-level)
        database.php
        session.php
        email.php
    /services (high-level)
        auth.php (dependent on database + session)
        invoicing.php (dependent on database + email)