symfony · weaverryan · Apr 11, 2012 · Mar 30, 2012 · Mar 30, 2012 · Mar 30, 2012
diff --git a/book/templating.rst b/book/templating.rst
@@ -1334,6 +1334,7 @@ Learn more from the Cookbook
 
 * :doc:`/cookbook/templating/PHP`
 * :doc:`/cookbook/controller/error_pages`
+* :doc:`/cookbook/templating/twig_extension`
 
 .. _`Twig`: http://twig.sensiolabs.org
 .. _`KnpBundles.com`: http://knpbundles.com

diff --git a/cookbook/index.rst b/cookbook/index.rst
@@ -71,6 +71,7 @@ Cookbook
 
     templating/global_variables
     templating/PHP
+    templating/twig_extension
 
     logging/monolog
     logging/monolog_email

diff --git a/cookbook/map.rst.inc b/cookbook/map.rst.inc
@@ -92,6 +92,7 @@
 
   * :doc:`/cookbook/templating/global_variables`
   * :doc:`/cookbook/templating/PHP`
+  * :doc:`/cookbook/templating/twig_extension`
 
 * **Logging**
 

diff --git a/cookbook/templating/twig_extension.rst b/cookbook/templating/twig_extension.rst
@@ -0,0 +1,128 @@
+.. index::
+   single: Twig extensions
+
+How to write a custom Twig Extension
+====================================
+
+The main motivation for writing an extension is to move often used code
+into a reusable class like adding support for internationalization. 
+An extension can define tags, filters, tests, operators, global variables,
+functions, and node visitors.
+
+Creating an extension also makes for a better separation of code that is
+executed at compilation time and code needed at runtime. As such, it makes
+your code faster.
+
+.. tip::
+
+    Before writing your own extensions, have a look at the `Twig official extension repository`_.
+
+Create the Extension Class
+--------------------------    
+
+To get your custom functionality you must first create a Twig Extension class. 
+As an example we will create a price filter to format a given number into price::
+
+    // src/Acme/DemoBundle/Twig/AcmeExtension.php
+
+    namespace Acme\DemoBundle\Twig;
+
+    use Twig_Extension;
+    use Twig_Filter_Method;
+    use Twig_Function_Method;
+
+    class AcmeExtension extends Twig_Extension
+    {
+        public function getFilters()
+        {
+            return array(
+                'price' => new Twig_Filter_Method($this, 'priceFilter'),
+            );
+        }
+
+        public function priceFilter($number, $decimals = 0, $decPoint = '.', $thousandsSep = ',')
+        {
+            $price = number_format($number, $decimals, $decPoint, $thousandsSep);
+            $price = '$' . $price;
+
+            return $price;
+        }
+
+        public function getName()
+        {
+            return 'acme_extension';
+        }
+    }
+
+.. tip::
+
+    Along with custom filters, you can also add custom `functions` and register `global variables`.    
+
+Register an Extension as a Service
+-------------------------------
+
+Now you must let Service Container know about your newly created Twig Extension:
+
+.. configuration-block::
+
+    .. code-block:: xml
+
+        <!-- src/Acme/DemoBundle/Resources/config/services.xml -->
+        <services>
+            <service id="acme.twig.acme_extension" class="Acme\DemoBundle\Twig\AcmeExtension">
+                <tag name="twig.extension" />
+            </service>
+        </services>
+
+    .. code-block:: yaml
+
+        # src/Acme/DemoBundle/Resources/config/services.yml
+        services:
+            acme.twig.acme_extension:
+                class: Acme\DemoBundle\Twig\AcmeExtension
+                tags:
+                    - { name: twig.extension }
+
+    .. code-block:: php
+
+        // src/Acme/DemoBundle/Resources/config/services.php
+        use Symfony\Component\DependencyInjection\Definition;
+
+        $acmeDefinition = new Definition('\Acme\DemoBundle\Twig\AcmeExtension');
+        $acmeDefinition->addTag('twig.extension');
+        $container->setDefinition('acme.twig.acme_extension', $acmeDefinition);
+
+.. note::
+
+   Keep in mind that Twig Extensions are not lazily loaded. This means that 
+   there's a higher chance that you'll get a **CircularReferenceException**
+   or a **ScopeWideningInjectionException** if any services 
+   (or your Twig Extension in this case) are dependent on the request service.
+   For more information take a look at :doc:`/cookbook/service_container/scopes`.
+
+Using the custom Extension
+--------------------------
+
+Using your newly created Twig Extension is no different than any other:
+
+.. code-block:: jinja
+
+    {# outputs $5,500.00 #}
+    {{ '5500' | price }}
+
+Passing other arguments to your filter:
+
+.. code-block:: jinja
+
+    {# outputs $5500,2516 #}
+    {{ '5500.25155' | price(4, ',', '') }}
+
+Learning further
+----------------
+
+For a more in-depth look into Twig Extensions, please take a look at the `Twig extensions documentation`_.
+
+.. _`Twig official extension repository`: http://github.com/fabpot/Twig-extensions
+.. _`Twig extensions documentation`: http://twig.sensiolabs.org/doc/extensions.html
+.. _`global variables`: http://twig.sensiolabs.org/doc/extensions.html#globals
+.. _`functions`: http://twig.sensiolabs.org/doc/extensions.html#functions