Composition

Composition occurs when one object “completely owns” another object. That is,

the first object was responsible for creating (instantiating) the second object.

There are many cases in which this can be useful, although, typically, composition

is used when it's likely that the first object will be the only one that needs to use

the second object.

One example from Volume II, Chapter 1 is the Auth class, which composes an

instance of the Session class, creating it in the constructor:

class Auth {

…

/**

* Instance of Session class

* @var Session

var $session;

…

function Auth (&$dbConn, $redirect, $md5 = true)

{

$this->dbConn = &$dbConn;

$this->redirect = $redirect;

$this->md5 = $md5;

$this->session = &new Session();

$this->checkAddress();

$this->login();

}

Because the Auth class needs to read and write to session variables, and only a

limited number of other, unrelated classes in an application are likely also to

need to use Session, it's logical that it gets to create its own Session object.