This book has been written for both symfony 1.3 and symfony 1.4. As writing a single book for two different versions of a software is quite unusual, this section explains what the main differences are between the two versions, and how to make the best choice for your projects.
Both the symfony 1.3 and symfony 1.4 versions have been released at about the same time (at the end of 2009). As a matter of fact, they both have the exact same feature set. The only difference between the two versions is how each supports backward compatibility with older symfony versions.
Symfony 1.3 is the release you’ll want to use if you need to upgrade a legacy project that uses an older symfony version (1.0, 1.1, or 1.2). It has a backward compatibility layer and all the features that have been deprecated during the 1.3 development period are still available. It means that upgrading is easy, simple, and safe.
If you start a new project today, however, you should use symfony 1.4. This version has the same feature set as symfony 1.3 but all the deprecated features, including the entire compatibility layer, have been removed. This version is cleaner and also a bit faster than symfony 1.3. Another big advantage of using symfony 1.4 is its longer support. Being a Long Term Support release, it will be maintained by the symfony core team for three years (until November 2012).
Of course, you can migrate your projects to symfony 1.3 and then slowly update your code to remove the deprecated features and eventually move to symfony 1.4 in order to benefit from the long term support. You have plenty of time to plan the move as symfony 1.3 will be supported for a year (until November 2010).
As this book does not describe deprecated features, all examples work equally well on both versions.

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Fabien Potencier