Adding a new entry about deprecation warnings

cookbook/map.rst.inc

@@ -224,6 +224,7 @@
   * :doc:`/cookbook/upgrade/patch_version`
   * :doc:`/cookbook/upgrade/minor_version`
   * :doc:`/cookbook/upgrade/major_version`
+  * :doc:`/cookbook/upgrade/deprecation_warnings`
 
 * :doc:`/cookbook/validation/index`
 

cookbook/upgrade/deprecation_warnings.rst

@@ -0,0 +1,74 @@
+What do these "XXX is deprecated " E_USER_DEPRECATED Warnings mean?
+===================================================================
+
+Starting in Symfony 2.7, if you use a deprecated class, function or option,
+Symfony triggers an ``E_USER_DEPRECATED`` error. Internally, that looks something
+like this::
+
+    trigger_error(
+        'The fooABC method is deprecated since version 2.4 and will be removed in 3.0.',
+        E_USER_DEPRECATED
+    );
+
+This is great, because you can check your logs to know what needs to change
+before you upgrade. In the Symfony Framework, the number of deprecated calls
+shows up in the web debug toolbar. And if you install the `phpunit-bridge`_,
+you can get a report of deprecated calls after running your tests.
+
+How can I Silence the Warnings?
+-------------------------------
+
+As useful as these are, you don't want them to show up while developing and
+you may also want to silence them on production to avoid filling up your
+error logs.
+
+In the Symfony Framework
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+In the Symfony Framework, ``~E_USER_DEPRECATED`` is added to ``app/bootstrap.php.cache``
+automatically, but you need at least version 2.3.14 or 3.0.21 of the
+`SensioDistributionBundle`_. So, you may need to upgrade:
+
+.. code-block:: bash
+
+    $ composer update sensio/distribution-bundle
+
+Once you've updated, the ``bootstrap.php.cache`` file is rebuilt automatically.
+At the top, you should see a line adding ``~E_USER_DEPRECATED``.
+
+Outside of the Symfony Framework
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To do that, add ``~E_USER_DEPRECATED`` to your ``error_reporting``
+setting in ``php.ini``:
+
+.. code-block:: ini
+
+    ; before
+    error_reporting = E_ALL
+    ; after
+    error_reporting = E_ALL & ~E_USER_DEPRECATED
+
+Alternatively, you can set this directly in bootstrap of your project::
+
+    error_reporting(error_reporting() & ~E_USER_DEPRECATED);
+
+How can I Fix the Warnings?
+---------------------------
+
+Of course ultimately, you want to stop using the deprecated functionality.
+Sometimes, this is easy: the warning might tell you exactly what to change.
+
+But other times, the warning might be unclear: a setting somewhere might
+cause a class deeper to trigger the warning. In this case, Symfony does its
+best to give a clear message, but you may need to research that warning further.
+
+And sometimes, the warning may come from a third-party library or bundle
+that you're using. If that's true, there's a good chance that those deprecations
+have already been updated. In that case, upgrade the library to fix them.
+
+Once all the deprecation warnings are gone, you can upgrade with a lot
+more confidence.
+
+.. _`phpunit-bridge`: https://github.com/symfony/phpunit-bridge
+.. _`SensioDistributionBundle`: https://github.com/sensiolabs/SensioDistributionBundle

cookbook/upgrade/index.rst

@@ -16,3 +16,4 @@ There are three types of upgrades, all needing a little different preparation:
     /cookbook/upgrade/patch_version
     /cookbook/upgrade/minor_version
     /cookbook/upgrade/major_version
+    /cookbook/upgrade/deprecation_warnings