php - When to use static vs instantiated classes

Question

PHP is my first programming language. I can't quite wrap my head around when to use static classes vs instantiated objects.

I realize that you can duplicate and clone objects. However in all of my time using php any object or function always ended up as a single return (array, string, int) value or void.

I understand concepts in books like a video game character class. duplicate car object and make the new one red, that all makes sense, but what doesn't is its application in php and web apps.

A simple example. A blog. What objects of a blog would be best implemented as static or instantiated objects? The DB class? Why not just instantiate the db object in the global scope? Why not make every object static instead? What about performance?

Is it all just style? Is there a proper way to do this stuff?

Answer 1

This is quite an interesting question -- and answers might get interesting too ^^

The simplest way to consider things might be :

• use an instanciated class where each object has data on its own (like a user has a name)
• use a static class when it's just a tool that works on other stuff (like, for instance, a syntax converter for BB code to HTML ; it doesn't have a life on its own)

(Yeah, I admit, really really overly-simplified...)

One thing about static methods/classes is that they don't facilitate unit testing (at least in PHP, but probably in other languages too).

Another thing about static data is that only one instance of it exists in your program : if you set MyClass::$myData to some value somewhere, it'll have this value, and only it, every where -- Speaking about the user, you would be able to have only one user -- which is not that great, is it ?

For a blog system, what could I say ? There's not much I would write as static, actually, I think ; maybe the DB-access class, but probably not, in the end ^^