Using objects in objects?

Is it possible to use an object within an object?

Yes.

Use the __construct function to initialize any sub-objects that you need

An answer that is very similar to the others, but with one - in my opinion, key - difference: rather than instantiating the dependency/dependencies in the constructor, pass them to the constructor:

class User { protected $db; public function __construct(Db $db) { $this->db = $db; } public function login($username, $password) { $this->db->query("SELECT * FROM users..."); } } 

The benefits to this type of dependency injection include:

1. It clearly demonstrates that the client/consumer class (User) has a dependency (Db).
2. Reduces coupling between the User class and the Db class. This helps significantly when unit-testing the User class. You can create a mock Db that performs as expected. Any test failures are then clearly attributable to the User class.

Of course, in order to instantiate a User object, you need to instantiate a Db object first. With a lot of dependencies for a single object or for a long chain of dependencies, this can be kind of a pain. In that case, it is helpful - but not required - to employ a Factory or a Dependency Injection Container to assist with this object creation.

Ofcource and you can, but not in that way. What you can do is like that:

class User { private $m = null; function __construct() { $this->m = new MySQL(); } function login($username, $password, $rememer = false) { $this->m->rQuery("SELECT * FROM users......"); } }