Adding a guide about upgrading

cookbook/upgrading.rst

@@ -0,0 +1,127 @@
+How to Upgrade your Symfony Project
+===================================
+
+So a new Symfony release has come out and you want to upgrade, great! Fortunately,
+because Symfony protects backwards-compatibility very closely, this *should*
+be quite easy.
+
+Upgrading a Patch Version (e.g. 2.6.0 to 2.6.1)
+-----------------------------------------------
+
+If you're upgrading and only the patch version (the last number) is changing,
+then it's *really* easy:
+
+.. code-block:: bash
+
+    $ composer update symfony/symfony
+
+That's it! You should not encounter any backwards-compatability breaks or
+need to change anything else in your code.
+
+You may also want to upgrade the rest of your libraries. If you've done a
+good job with your version constraints in ``composer.json``, you can do this
+safely by running:
+
+.. code-block:: bash
+
+    $ composer update symfony/symfony
+
+But beware. If you have some bad version constraints in your ``composer.json``,
+(e.g. ``dev-master``), then this could upgrade some non-Symfony libraries
+to new versions that contain backwards-compability changes.
+
+Upgrading a Minor Version (e.g. 2.5.3 to 2.6.0)
+-----------------------------------------------
+
+If you're upgrading a minor version (where the middle number changes), then
+you should also *not* encounter significant backwards compability changes.
+For details, see our :doc:`/contributing/code/bc`.
+
+However, some backwards-compability breaks *are* possible, and you'll learn
+in a second how to prepare for them.
+
+There are two steps to upgrading:
+
+1. :ref:`upgrade-minor-symfony-composer`;
+2. :ref:`upgrade-minor-symfony-code`, which includes instructions for each version.
+
+.. _`upgrade-minor-symfony-composer`:
+
+Update the Symfony Library
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+First, you need to update Symfony by modifying your ``composer.json`` to
+use the new version:
+
+.. code-block:: json
+
+    {
+        "...": "...",
+
+        "require": {
+            "php": ">=5.3.3",
+            "symfony/symfony": "~2.6.0",
+            "...": "... no changes to anything else..."
+        },
+        "...": "...",
+    }
+
+Next, update the same as before:
+
+.. code-block:: bash
+
+    $ composer update symfony/symfony
+
+Updating a minor version like this should *not* cause any dependency issues,
+though it's always possible that an outside library or bundle you're using
+didn't support this new version of Symfony at the version you have of that
+library.  In that case, consult the library: you may need to modify its version
+in ``composer.json`` and run a full ``composer update``. 
+
+.. _`upgrade-minor-symfony-code`:
+
+Updating your Code to work with the new Version
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In theory, you should be done! However, you *may* need to make a few changes
+to your code to get everything working. Additionally, some features you're
+using might still work, but might now be deprecated. That's actually ok,
+but if you know about these deprecations, you can start to fix them over
+time.
+
+Every version of Symfony comes with an UPGRADE file that describes these
+changes. Below are links to the file for each version, along with some other
+details.
+
+Upgrading to Symfony 2.6
+........................
+
+First, of course, update your ``composer.json`` file with the ``2.6`` version
+of Symfony as described above in :ref:`upgrade-minor-symfony-composer`.
+
+Check the `UPGRADE-2.6`_ document for details. Highlights:
+
+* If you're using PdoSessionStorage, there was a change in the session schema
+  that **requires** your session table to be updated. See :doc:`/cookbook/configuration/pdo_session_storage`.
+
+* Symfony 2.6 comes with a great new `dump`_ function. To use it, you'll
+  need to add the new ``DebugBundle`` to your ``AppKernel``. See
+  `UPGRADE-2.6-DebugBundle`_ for details.
+
+Upgrading to Symfony 2.5
+........................
+
+First, of course, update your ``composer.json`` file with the ``2.5`` version
+of Symfony as described above in :ref:`upgrade-minor-symfony-composer`.
+
+Check the `UPGRADE-2.5`_ document for details. Highlights:
+
+* This version introduced a new Validator API. But, as long as you're using
+  PHP 5.3.9 or higher, you can configure Symfony in a way that allows you
+  to use the new API, but still let the old API work (called ``2.5-bc``).
+  See the `UPGRADE-2.5-Validator`_ for details.
+
+.. _`UPGRADE-2.5`: https://github.com/symfony/symfony/blob/2.5/UPGRADE-2.5.md
+.. _`UPGRADE-2.5-Validator`: https://github.com/symfony/symfony/blob/2.7/UPGRADE-2.5.md#validator
+.. _`UPGRADE-2.6`: https://github.com/symfony/symfony/blob/2.6/UPGRADE-2.6.md
+.. _`UPGRADE-2.6-DebugBundle`: https://github.com/symfony/symfony/blob/2.6/UPGRADE-2.6.md#vardumper-and-debugbundle