This article is part of a series of articles that explains how to create a framework with the Symfony Components. It is OBSOLETE but an up-to-date version can be found in the Symfony documentation.

Symfony2 is a reusable set of standalone, decoupled, and cohesive PHP components that solve common web development problems.

Instead of using these low-level components, you can use the ready-to-be-used Symfony2 full-stack web framework, which is based on these components... or you can create your very own framework. This series is about the latter.

If you just want to use the Symfony2 full-stack framework, you'd better read its official documentation instead.

Why would you like to create your own framework?#

Why would you like to create your own framework in the first place? If you look around, everybody will tell you that it's a bad thing to reinvent the wheel and that you'd better choose an existing framework and forget about creating your own altogether. Most of the time, they are right but I can think of a few good reasons to start creating your own framework:

To learn more about the low level architecture of modern web frameworks in general and about the Symfony2 full-stack framework internals in particular;

To create a framework tailored to your very specific needs (just be sure first that your needs are really specific);

To experiment creating a framework for fun (in a learn-and-throw-away approach);

To refactor an old/existing application that needs a good dose of recent web development best practices;

To prove the world that you can actually create a framework on your own (... but with little effort).

I will gently guide you through the creation of a web framework, one step at a time. At each step, you will have a fully-working framework that you can use as is or as a start for your very own. We will start with simple frameworks and more features will be added with time. Eventually, you will have a fully-featured full-stack web framework.

And of course, each step will be the occasion to learn more about some of the Symfony2 Components.

If you don't have time to read the whole series, or if you want to get started fast, you can also have a look at Silex, a micro-framework based on the Symfony2 Components. The code is rather slim and it leverages many aspects of the Symfony2 Components.

Many modern web frameworks call themselves MVC frameworks. We won't talk about MVC here as the Symfony2 Components are able to create any type of frameworks, not just the ones that follow the MVC architecture. Anyway, if you have a look at the MVC semantics, this series is about how to create the Controller part of a framework. For the Model and the View, it really depends on your personal taste and I will let you use any existing third-party libraries (Doctrine, Propel, or plain-old PDO for the Model; PHP or Twig for the View).

When creating a framework, following the MVC pattern is not the right goal. The main goal should be the Separation of Concerns; I actually think that this is the only design pattern that you should really care about. The fundamental principles of the Symfony2 Components are centered around the HTTP specification. As such, the frameworks that we are going to create should be more accurately labelled as HTTP frameworks or Request/Response frameworks.

Before we start#

Reading about how to create a framework is not enough. You will have to follow along and actually type all the examples we will work on. For that, you need a recent version of PHP (5.3.8 or later is good enough), a web server (like Apache or NGinx), a good knowledge of PHP and an understanding of Object Oriented programming.

Ready to go? Let's start.

Before we can even think of creating our first framework, we need to talk about some conventions: where we will store our code, how we will name our classes, how we will reference external dependencies, etc.

To store our framework, create a directory somewhere on your machine:

$ mkdir framework $ cd framework

Coding Standards#

Before anyone starts a flame war about coding standards and why the one used here suck hard, let's all admit that this does not matter that much as long as you are consistent. For this book, we are going to use the Symfony2 Coding Standards.

Components Installation#

To install the Symfony2 Components that we need for our framework, we are going to use Composer, a project dependency manager for PHP. First, list your dependencies in a composer.json file:

{ "require": { "symfony/class-loader": "2.1.*" } }

Here, we tell Composer that our project depends on the Symfony2 ClassLoader component, version 2.1.0 or later. To actually install the project dependencies, download the composer binary and run it:

$ wget http://getcomposer.org/composer.phar $ # or $ curl -O http://getcomposer.org/composer.phar $ php composer.phar install

After running the install command, you must see a new vendor directory that must contain the Symfony2 ClassLoader code.

Even if we highly recommend you the use of Composer, you can also download the archives of the components directly or use Git submodules. That's really up to you.

Naming Conventions and Autoloading#

We are going to autoload all our classes. Without autoloading, you need to require the file where a class is defined before being able to use it. But with some conventions, we can just let PHP do the hard work for us.

Symfony2 follows the de-facto PHP standard, PSR-0, for class names and autoloading. The Symfony2 ClassLoader Component provides an autoloader that implements this PSR-0 standard and most of the time, the Symfony2 ClassLoader is all you need to autoload all your project classes.

Create and empty autoloader in a new autoload.php file:

register();

You can now run the autoload.php on the CLI, it should not do anything and should not throw any error:

$ php autoload.php

The Symfony website has more information about the ClassLoader component.

Composer automatically creates an autoloader for all your installed dependencies; instead of using the ClassLoader component, you can also just require vendor/.composer/autoload.php .

Our Project#

Instead of creating our framework from scratch, we are going to write the same "application" over and over again, adding one abstraction at a time. Let's start with the simplest web application we can think of in PHP:

That's all for the first part of this series. Next time, we will introduce the HttpFoundation Component and see what it brings us.