Pimple is a small Dependency Injection Container for PHP 5.3 that consists of just one file and one class (about 80 lines of code).

Download it, require it in your code, and you're good to go:

require_once '/path/to/Pimple.php';

Creating a container is a matter of instating the Pimple class:

$container = new Pimple();

As many other dependency injection containers, Pimple is able to manage two different kind of data: services and parameters.

Defining a parameter is as simple as using the Pimple instance as an array:

// define some parameters
$container['cookie_name'] = 'SESSION_ID';
$container['session_storage_class'] = 'SessionStorage';

A service is an object that does something as part of a larger system. Examples of services: Database connection, templating engine, mailer. Almost any object could be a service.

Services are defined by anonymous functions that return an instance of an object:

// define some services
$container['session_storage'] = function ($c) {
    return new $c['session_storage_class']($c['cookie_name']);
};

$container['session'] = function ($c) {
    return new Session($c['session_storage']);
};

Notice that the anonymous function has access to the current container instance, allowing references to other services or parameters.

As objects are only created when you get them, the order of the definitions does not matter, and there is no performance penalty.

Using the defined services is also very easy:

// get the session object
$session = $container['session'];

// the above call is roughly equivalent to the following code:
// $storage = new SessionStorage('SESSION_ID');
// $session = new Session($storage);

By default, each time you get a service, Pimple returns a new instance of it. If you want the same instance to be returned for all calls, wrap your anonymous function with the share() method:

$container['session'] = $container->share(function ($c) {
    return new Session($c['session_storage']);
});

Because Pimple sees anonymous functions as service definitions, you need to wrap anonymous functions with the protect() method to store them as parameter:

$container['random'] = $container->protect(function () { return rand(); });

In some cases you may want to modify a service definition after it has been defined. You can use the extend() method to define additional code to be run on your service just after it is created:

$container['mail'] = function ($c) {
    return new \Zend_Mail();
};

$container['mail'] = $container->extend('mail', function($mail, $c) {
    $mail->setFrom($c['mail.default_from']);
    return $mail;
});

The first argument is the name of the object, the second is a function that gets access to the object instance and the container. The return value is a service definition, so you need to re-assign it on the container.

If the service you plan to extend is already shared, it's recommended that you re-wrap your extended service with the shared method, otherwise your extension code will be called every time you access the service:

$container['twig'] = $container->share(function ($c) {
    return new Twig_Environment($c['twig.loader'], $c['twig.options']);
});

$container['twig'] = $container->share($container->extend('twig', function ($twig, $c) {
    $twig->addExtension(new MyTwigExtension());
    return $twig;
}));

When you access an object, Pimple automatically calls the anonymous function that you defined, which creates the service object for you. If you want to get raw access to this function, you can use the raw() method:

$container['session'] = $container->share(function ($c) {
    return new Session($c['session_storage']);
});

$sessionFunction = $container->raw('session');

If you use the same libraries over and over, you might want to create reusable containers. Creating a reusable container is as simple as creating a class that extends Pimple, and configuring it in the constructor:

class SomeContainer extends Pimple
{
    public function __construct()
    {
        $this['parameter'] = 'foo';
        $this['object'] = function () { return stdClass(); };
    }
}

Using this container from your own is as easy as it can get:

$container = new Pimple();

// define your project parameters and services
// ...

// embed the SomeContainer container
$container['embedded'] = $container->share(function () { return new SomeContainer(); });

// configure it
$container['embedded']['parameter'] = 'bar';

// use it
$container['embedded']['object']->...;