How can I use "Dependency Injection" in simple php functions, and should I bother?

I hear people talking about dependency injection and the benefit of it all the time, but I don't really understand it.

I'm wondering if it's a solution to the "I pass database connections as arguments all the time" problem.

I tried reading wikipedia's entry on it, but the example is written in Java so I don't solidly understand the difference it is trying to make clear. ( http://en.wikipedia.org/wiki/Dependency_injection ).

I read this dependency-injection-in-php article ( http://www.potstuck.com/2009/01/08/php-dependency-injection/ ), and it seems like the objective is to not pass dependencies to an object directly, but to cordon off the creation of an object along with the creation of it's dependencies. I'm not sure how to apply that in a using php functions context, though.

Additionally, is the following Dependency Injection, and should I bother trying to do dependency injection in a functional context?

Version 1: (the kind of code that I create, but don't like, every day)

function get_data_from_database($database_connection){
    $data = $database_connection->query('blah');
    return $data;
}

Version 2: (don't have to pass a database connection, but perhaps not dependency injection?)

function get_database_connection(){
    static $db_connection;
    if($db_connection){
        return $db_connection;
    } else {
        // create db_connection
      ...
    }
}

function get_data_from_database(){
   $conn = get_database_connection();
   $data = $conn->query('blah');
   return $data;
}

$data = get_data_from_database();

Version 3: (the creation of the "object"/data is separate, and the database code is still, so perhaps this would count as dependency injection?)

function factory_of_data_set(){
    static $db_connection;
    $data_set = null;
    $db_connection = get_database_connection();
    $data_set = $db_connection->query('blah');
    return $data_set;
}

$data = factory_of_data_set();

Anyone have a good resource or just insight that makes the method and benefit -crystal- clear?

Dependency injection is a big word for "I have some more parameters in my constructor".

It's what you did before the awfull Singleton wave when you did not like globals :

<?php
class User {
    private $_db;
    function __construct($db) {
        $this->_db = $db;
    }
}

$db   = new Db();
$user = new User($db);

Now, the trick is to use a single class to manage your dependencies, something like that :

class DependencyContainer 
{
    private _instances = array();
    private _params = array();

    public function __construct($params)
    {
        $this->_params = $params;
    }

    public function getDb()
    {
        if (empty($this->_instances['db']) 
            || !is_a($this->_instances['db'], 'PDO')
        ) {
            $this->_instances['db'] = new PDO(
                $this->_params['dsn'],
                $this->_params['dbUser'], 
                $this->_params['dbPwd']
            );
        }
        return $this->_instances['db'];
    }
}

class User
{
    private $_db;
    public function __construct(DependencyContainer $di)
    {
         $this->_db = $di->getDb();
    }
}

$dependencies = new DependencyContainer($someParams);
$user = new User($dependencies);

You must think you just another class and more complexity. But, your user class may need something to log messages like lot of other classes. Just add a getMessageHandler function to your dependency container, and some $this->_messages = $di->getMessageHandler() to your user class. Nothing to change in the rest of your code.

You'll get lot of infos on symfony's doc

Your first example IS dependancy injection, you are injecting the dependency on the database object into the function.

Sarah has said this isn't, but imo it is, I believe she is thinking of dependency injection containers which are the next level up:

http://components.symfony-project.org/dependency-injection/trunk/book/02-Dependency-Injection-Containers

None of your examples look like dependency injection, version one is the closest though. Dependency injection is a technique used in object oriented programming, where the constructor of an object has arguments for the service objects it needs, and those service objects are passed in by the creator of the instance (which could be a factory, a test, or a dependency injection framework).

To get around your 'always passing the connection object' problem you may want to consider the template pattern. The template pattern is basically an abstract base class with the common part of a repeated code block, and abstract methods to allow for the variation between the instances of those repeated code blocks. Basically the base is a template of a block of code, and the abstract methods are the blanks to be filled in. I personally use the template method pattern to do my database resource control in Java.

I have done much searching on this topic myself (PHP Dependency Injection) and haven't found much to my liking. A lot has been written on the subject for other languages (Google Guice - http://code.google.com/p/google-guice/ ; Java Spring), but I couldn't find much available for PHP. Regardless of the language, however, the challenges are similar.

The three versions you list in your question are the typical approach. Version 3 is the closest to the direction in which I have seen the industry going. By shifting the responsibility of creating your dependent objects outside of your class, you are free to manipulate them as you please in your test code. However, the problem that I encountered with that approach is that you end up with long chains of dependent objects in your constructor that can potentially not even be used by the receiving object, but get passed through to an secondary dependent object. It gets messy and you lose knowledge of what is coming from where.

The Dependency Container example by @Arkh and @mmmshuddup is a great start, but I nonetheless found limitations with that approach as well. The final solution upon which I arrived was a custom built solution modeled somewhat after the Cake Pattern popular in Scala. It allows you to pass a single dependency into each of your constructors AND it lets you define the default construction of the dependent objects per class. This frees you from long dependency chains as well as losing control of the default implementations of your dependencies.

I called the system Diesel and I've been really happy with it. I published the code on github for anyone interested. You can get to it from the blog I wrote on the subject, which describes the basic usage as well as goes into more detail on your question. http://developers.blog.box.com/2012/02/15/introducting-diesel-php-dependency-injection/

Dependency Injection is the idea of removing the dependency between 2 components in order to focus on why they are dependent.

Imagine you have a component A that needs to use the services of another component B.

If you hardcode the existence of B inside A, then you will be stuck when you will want A to use the sames services, but implemented by another component.

So usually, you define a service interface that B and C will implement, and you make sure that when you use A, you feed it with objects compatible with the needed interface.

In your case, you might consider that your interface is a service on which you can make a query.

Your first case is the one that is the closer to the idea of Dependency Injection.