How do I declare 'sub-objects' in PHP

Question

I'm relatively new to OOP in PHP, and I'm not sure if what I'm trying to do is possible or recommended. In any case, I can't figure it out. I'd appreciate any pointers to tutorials or documents which might help - I'm not expecting a full-blown answer here.

I have a system in which each user has a number of 'Libraries'. Each Library contains a number of 'Elements'.

DB set up is as follows:

user_libraries
 - id (unique)
 - user_id (identifies user)
 - name (just a string)

elements
 - id (unique)
 - content (a string)

library_elements
 - id (unique)
 - library_id
 - element_id

where library_id is the id from user_libraries, and element_id is that from elements.

I want to be able to access a given user's library, and their elements.

I've set up the library class, and can use it to retrieve the list of libraries (or a sub-list).

I do this like this:

$mylibraryset = new LibrarySet();
$mylibraryset->getMyLibraries();

which gives (when I use print_r):

LibrarySetObject (
 [user_id] => 105
 [data_array] => Array (
  [0] => Array (
   [id] => 1
   [user_id] => 105
   [type] => 1
   [name] => My Text Library
  )
  [1] => Array (
   [id] => 2
   [user_id] => 105
   [type] => 2
   [name] => Quotes
  )
 )
)

Now, what I'd like to be able to do is for each of those libraries (the elements in data_array), to retrieve all the elements.

The best idea I've had so far is to do something like:

foreach($mylibrary->data_array as $library) {
 $sublibrary = new Library();
 $sublibrary -> getAllElements();
}

where Sublibrary is another class which has the function getAllElements. I can't quite get it to work though, and I'm not sure I'm on the right lines...

Is there a way that I can then end up being able to do something like this:

$mylibrary->sublibraries[0]->element[0]

to retrieve a specific element?

As I say, I don't expect a full-blown explanation here - just pointers to get me started.

Answer 1

<?php

class Library {
    public $element;
    public $data;
    public function __construct($sublibrary) {
        $this->data = $sublibrary;
    }
    public function getAllElements() {
        // populate $this->element using $this->data
    }
}

class LibrarySet {
    public $user_id;
    public $data_array;
    public $sublibraries;
    public function getMyLibraries() {
        // populate $this->data_array

        $this->sublibraries = Array();
        foreach($this->data_array as $index => $sublibrary) {
            $this->sublibraries[$index] = new Library($sublibrary);
            $this->sublibraries[$index]->getAllElements();
        }
    }
}

$mylibraryset = new LibrarySet();
$mylibraryset->getMyLibraries();

$mylibraryset->sublibraries[0]->element[0]

?>