Question

I’m expanding my php and general coding knowledge by writing my own ‘simple’ CMS system.

My database structure follows:

USERS --< WEBSITES --< PAGES

In my logic, I have three classes, a User, Website and Page class. NOT INHERITED

The User class has all the attributes from the USER TABLE plus a $_website attribute,

The Website class has all the attributes of the WEBSITE TABLE plus an ARRAY of Page objects

The way I have decided to work this is, on page load, these classes will be created and filled with the database data.

So then in the html I can echo out results simply by calling the class like so:

$USER -> name;
$USER -> website -> get_pages();
$USER -> website -> pages[0] -> get_header();

---

I Realised that these classes could be inherited, so Page inherits Website and Website inherits User

But if I do it that way, then I’ll instantiate the Page class like:

$PAGES = array(); 
$PAGES[] = new Page( $constructor_data_to_construct_user_website_and_page );

Now IF I do it this way, the object $PAGES[0] will have all the info to do with that page, and all the info to do with the Website (first parent),
and all the info to do with the User ( second parent )

If i then create another Page, i’m just copying the same User and Website details Yes?

So if i were to var_dump $PAGES[0] id get ALL the info to do with USER, WEBSITE and PAGE

If i were to var_dump $PAGES[1] id get ALL THE SAME information for USERS AND WEBSITE, but just the different Page data.

So inheritance, is causing unnecessary copying of the User and Website objects

Because I could essentially do:

$PAGES[0] -> username // username is a property of the Users class
$PAGES[1] -> username

both would return EXACTLY the same value.

surely this is bad because data is being unnecessarily copied!

Is This Correct?

Or is this question a load of rubbish and this isn’t the kind of situation inheritance is used?

Answer 1

This is exactly the sort of situation where you must not use inheritance.

Inheritance is meant to model an IS-A relationship. Typical example: a Student is a type of Person, so Student inherits from Person.

Inheritance is not, not, NOT meant to model a HAS-A relationship. Typical example: a Car has an Engine, but Car is not a type of Engine.

When we have a HAS-A relationship (e.g. a User has Website) you should almost in all cases use composition instead of inheritance:

Bad

class Website extends User {}

Good

class Website {
    private $users;
}