PHP中的依赖注入困惑（类的链）

Question

I'm moving from Globals and Singletons (=bad?) to Dependency Injections (=good?) in PHP and I'm very new to it.

I have read many related topics on Stack Overflow already but I still can't understand the main principles of DI.

Please tell me if I'm doing it right (it's just shortened pseudo-code):

// first connect to DB
$sql = new Sql(); 

// create logger (I'm writing logs to Database so I need to pass $sql to it)
$log = new Log($sql);

// restore logged in user, get info about user, unread messages... 
// class "User" needs  access to Database and Logs:
$user = new User($sql, $log); 

// now we need to collect all the data of current section of my Website
// I'm using Model and Controller:
$model = new FrontPageModel($sql, $user, $log); 

$pageController = new FrontPageController($model);

Though it may look OK at this step but what if I need to get access to more classes like Config, Session, etc.?

Should my code transform to this?

$model = new FrontPageModel($sql, $user, $log, $config, $session); 

Isn't it too much already?

I know someone could advice to use a kind of big "Application" class and put Config, Session, Log, Db objects inside of this class but I feel that it isn't very good idea.

Next question - what if I need to get logged User's ID inside of my FrontPageController? I did not pass an instance of "User" to FrontPageController, but it was passed before (in chain) to FronPageModel.

class FrontPageController{
  private $model;
  function __construct($model){
    $this->model = $model;
  }

  function getData(){
    echo $this->model->user->id; // is it right way?
  }
}

That "$this->model->user->id" seems like overkill to me.

Answer 1

It might not be the prettiest, but you're certainly not doing it "wrong". You're demonstrating classing constructor injection, but you could perhaps refactor to keep some of the disparate objects separated a bit more.

My suggestion would be to look at established PHP DI containers. See how they work, use them in a few apps, and (1) you'll have much more testable apps and (2) you'll be much more comfortable with DI in general.

Take a look at one or more of the following:

• Pimple
• Aura DI
• Symfony DI