minor #10593 Update twig_extension.rst (ThomasLandauer)

This PR was squashed before being merged into the 2.8 branch (closes #10593).

Discussion
----------

Update twig_extension.rst

Added example for custom function

Commits
-------

32e3179 Update twig_extension.rst

Author: javiereguiluz

templating/twig_extension.rst

@@ -41,11 +41,11 @@ As an example you'll create a price filter to format a given number into price::
         public function getFilters()
         {
             return array(
-                new TwigFilter('price', array($this, 'priceFilter')),
+                new TwigFilter('price', array($this, 'formatPrice')),
             );
         }
 
-        public function priceFilter($number, $decimals = 0, $decPoint = '.', $thousandsSep = ',')
+        public function formatPrice($number, $decimals = 0, $decPoint = '.', $thousandsSep = ',')
         {
             $price = number_format($number, $decimals, $decPoint, $thousandsSep);
             $price = '$'.$price;
@@ -62,9 +62,32 @@ As an example you'll create a price filter to format a given number into price::
     versions before 1.26, include this method which is omitted in the example
     above.
 
+Here's how to create a custom **function**::
+
+    // src/AppBundle/Twig/AppExtension.php
+    namespace AppBundle\Twig;
+
+    use Twig\Extension\AbstractExtension;
+    use Twig\TwigFunction;
+
+    class AppExtension extends AbstractExtension
+    {
+        public function getFunctions()
+        {
+            return array(
+                new TwigFunction('total', array($this, 'calculateArea')),
+            );
+        }
+
+        public function calculateArea(int $width, int $length)
+        {
+            return $width * $length;
+        }
+    }
+
 .. tip::
 
-    Along with custom filters, you can also add custom `functions`_ and register
+    Along with custom filters and functions, you can also register
     `global variables`_.
 
 Register an Extension as a Service