diff --git a/src/Symfony/Component/HttpFoundation/SessionStorage/ArraySessionStorage.php b/src/Symfony/Component/HttpFoundation/SessionStorage/ArraySessionStorage.php
index 43f1f1940f..ae14fdf07e 100644
--- a/src/Symfony/Component/HttpFoundation/SessionStorage/ArraySessionStorage.php
+++ b/src/Symfony/Component/HttpFoundation/SessionStorage/ArraySessionStorage.php
@@ -12,7 +12,9 @@
 namespace Symfony\Component\HttpFoundation\SessionStorage;
 
 /**
- * ArraySessionStorage.
+ * ArraySessionStorage mocks the session for unit tests.
+ *
+ * When doing functional testing, you should use FilesystemArrayStorage instead.
  *
  * @author Fabien Potencier <fabien@symfony.com>
  * @author Bulat Shakirzyanov <mallluhuct@gmail.com>