How to build a PHP Dependency Injection Container

Question

I've recently learned about the advantages of using Dependency Injection (DI) in my PHP application. However, I'm still unsure how to create my container for the dependencies, or whether I should be using DI at all for the online forum that I'm building.

The following code is my version of the DI container I have made based on the example I learned from here .

class ioc {

   var $db;
   var $session;
   var $user_id;

   static function newUser(static::$db, static::$user_id) {
      $user = new User($db, $user_id);
      return $user;
   }

   static function newLogin(static::$db, static::$session) {
      $login = new Login($db, $session);
      return $login;
   }

}

$user = ioc::newUser();
$login = ioc::newLogin();

I have a few questions:

1) Where should I instantiate my injected dependencies, such as $database, $session, etc? Would it be outside the container class, or inside the container's constructor.

2) What if I need to create a multiple instances of the User class inside other classes? I can't inject the previously instantiated $user object because that instance is already being used. However, creating the multiple User instances inside of another class would violate the rules of DI. For example:

class Users {

    function __construct($db, $user_id) {
        $this->db = $db;
        $this->user_id = $user_id;
    }

    function create_friends_list() {
        $st = $this->$db->prepare("SELECT user_id FROM friends WHERE user_id = $this->user_id");
        $st->execute();
        while($row = $st->fetch()) {
            $friend = ioc::newUser($row['user_id']);
            $friend->get_user_name();
            $friend->get_profile_picture();
        }
    }
}   

3) I'm wondering if I should even adopt DI, knowing that I have to rewrite all of my previous code. I've previously been relying on global variables that I instantiate in my initialize.php, which is included in all my files.

It seems to me that DI creates a lot of overhead and there are situations where it is unusable (as in my #2 example). The following site is from a developer who cites many good reasons not to use DI. Does his arguments have any merit? Or am I just using DI wrong? check this link.

Answer 1

Where should I instantiate my injected dependencies, such as $database, $session, etc? Would it be outside the container class, or inside the container's constructor.

Ideally your database connection and session would be bootstrapped in. Proper DI requires an instance of a base object for which everything is registered into. So taking your IOC class as an example you need to make an instance of it ($ioc = new IOC();) then you need some kind of service provider class say

$ioc->register('database', new DatabaseServiceProvider($host, $user, $pass))

Now every time you want a connection to the database you just need to pass in $ioc->get('database'); a very rough example but I think you can see the idea is basically to store everything inside a registry and nothing is statically binded meaning you can create another instance of $ioc with totally different settings making it easy to create connections to say a different database for testing purposes.

What if I need to create a multiple instances of the User class inside other classes? I can't inject the previously instantiated $user object because that instance is already being used. However, creating the multiple User instances inside of another class would violate the rules of DI.

This is a common issue and there are multiple different solutions. Firstly your DI should show the difference between logged in user and just a user. You would probably want to register your logged in user but not just any user. make your user class just normal and use

$ioc->register('login-user', User::fetch($ioc->get('database'), $user_id));

so now $ioc->get('login-user') returns your logged in user. You can then use User->fetchAll($ioc->get('database')); to get all your users.

I'm wondering if I should even adopt DI, knowing that I have to rewrite all of my previous code. I've previously been relying on global variables that I instantiate in my initialize.php, which is included in all my files.

If you need to rewrite all your code to use DI you shouldn't probably do it. Maybe look into making a new project and work in some of your old code if you have the time. If your codebase is large I would recommend looking into breaking it down into smaller projects and using RESTFUL apis for getting and saving data. Good examples of writing APIs would be for putting your forum search into its own application /search/name?partial-name=bob would return all users with the word bob in it. you could build it up and make it better over time and use it in your main forum

I hope you understand my answers but if you need any more info let me know.

Answer 2

I was going to write this a comment, but it grew too long. I am not an expert so I will just give my point of view from what I've learned through few years practicing and here in SO. Feel free to use or question any part of my answer (or none).

1.- Outside. What does the container do? The answer should be a single thing. It shouldn't have to be responsible to initialize the classes, connect to the database, handle the session and other things. Each class does one thing only.

class ioc
  {
  public $db;

  // Only pass here the things that the class REALLY needs
  static public function set($var, $val)
    {
    return $this->$var = $val;
    }

  static function newDB($user, $pass)
    {
    return new PDO('mysql:host=localhost;dbname=test', $user, $pass);
    }

  static function newUser($user_id)
    {
    return new User($db, $user_id);
    }

  static function newLogin($session)
    {
    return new Login($this->db, $session);
    }
  }

if (ioc::set('db',ioc::newDB($user, $pass)))
  {
  $user = ioc::newUser($user_id);
  $login = ioc::newLogin($session);
  }

2.- You shouldn't do $friend = ioc::newUser($row['user_id']); inside your class. There you are assuming that there's a class called ioc with a method called newUser(), while each class should be able to act on it's own, not based on [possibly] other existing classes. This is called tight coupling. Basically, that's why you shouldn't use global variables either. Anything used within a class should be passed to it, not assumed in the global scope. Even if you know it's there and your code works, it makes the class not reusable for other projects and much harder to test. I will not extend myself (PUN?) but put a great video I discovered here in SO so you can dig more: The Clean Code Talks - Don't Look For Things.

I'm not sure about how the class User behaves, but this is how I'd do it (not necessary right):

// Allow me to change the name to Friends to avoid confusions
class Friends
  {
  function __construct($db)
    {
    $this->db = $db;
    }

  function create_friends_list($user_id)
    {
    if (!empty(id))
      {
      // Protect it from injection if your $user_id MIGHT come from a $_POST or whatever
      $st = $this->$db->prepare("SELECT user_id FROM friends WHERE user_id = ?");
      $st->execute(array($user_id));
      $AllData = $st->fetchAll()
      return $AllData;
      }
    else return null;
    }

  // Pass the $friend object
  function get_friend_data($friend)
    {
    $FriendData = array ('Name' => $friend->get_user_name(), 'Picture' => $friend->get_profile_picture());
    return $FriendData;
    }
  }

$User = ioc::newUser($user_id);
$Friends = new Friends($db);

$AllFriendsIDs = array();
if ($AllFriendsIDs = $Friends->create_friends_list($User->get('user_id')))
  foreach ($AllFriendsIDs as $Friend)
    {
    // OPTION 1. Return the name, id and whatever in an array for the user object passed.
    $FriendData = $Friends->get_friend_data(ioc::newUser($Friend['user_id']));
    // Do anything you want with $FriendData

    // OPTION 2. Ditch the get_friend_data and work with it here directly. You're already in a loop.
    // Create the object (User) Friend.
    $Friend = ioc::newUser($Friend['user_id']);
    $Friend->get_user_name();
    $Friend->get_profile_picture();
    }

I didn't test it, so it has probably some small bugs.

3.- If you are learning while coding, you will have to rewrite MANY things. Try to do somethings right from the beginning so you don't need to rewrite everything, but only the classes/methods and adopting some conventions for all your code. For example, never echo from within the function/method, always return and echo from outside. I'd say that yes, it's worth it. It's bad that you have to loose 1 or 2 hours just rewriting something, but if it has to be done, do it.

PS, sorry, I changed your bracket style everywhere.

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EDIT Reading other answers, while you shouldn't connect to the database with your ioc object, it should be perfeclty fine create a new object with it. Edited above to see what I mean.