I'm developing a CMS for a client where he can only access it with a password. In this site he can edit, delete, create new articles. I use this system:
The problem is that with the "action" files. for action files I mean the files that receive the forms data and requests and send them to the classes to handle. Last time I made a single file with a switch with for example "delete_article", "create_article" etc with the relative method to call but this becomes easily a MESS. I was wondering if there could be simplest ways to organize this. Maybe just doing php file for every "action" I wanna do?
Please considering I'm just starting with OOP so I could have said a lot stupid things :D
You may consider using a framework such as Codeigniter if you are just starting with OOP. The learning curve for good OOP is steep, but the learning curve to Codeigniter is not. You can start work immediately and will learn a lot of useful best practices on the way. Such as the MVC pattern, which is what you want to use when working with OOP. Head over to codeigniter.com and check it out!
The manual is available at http://codeigniter.com/user_guide/
If I was to go straight at answering your question, I would suggest you have a closer look at MVC which is a way of organizing files. But I know that this may be hard to understand in the beginning, which is why I tell people to introduce themselves to it by using a framework of their choice. Of course, from a development perspective having a framework is also preferable because it will speed up your process.
Like Razor said in his comment, reinventing the wheel may be a waste of time. You'll also be happy to note, then, that there is plenty of open-source code written in specific frameworks and if you know these frameworks, you will be able to extend on that code very easily. If you try to extend upon a random CMS-script, this may very well be a waste of time as well if you can't understand it.
You should consider take a look at the MVC. Even if you are OOP beginner it should be not so difficult to understand the idea.
And what would really help you is its (MVC) implementations in PHP.
Some very good tutorials about:
http://anantgarg.com/2009/03/13/write-your-own-php-mvc-framework-part-1/
http://php-html.net/tutorials/model-view-controller-in-php/
http://www.phpro.org/tutorials/Model-View-Controller-MVC.html
And speaking in the context of MVC you will eventually come up with organizing all of your create, delete and insert functionality into a different methods (actions) in your controllers.