diff --git a/reference/session/reference.xml b/reference/session/reference.xml
index c7e0a55298..cdcee19f20 100644
--- a/reference/session/reference.xml
+++ b/reference/session/reference.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.49 $ -->
+<!-- $Revision: 1.50 $ -->
  <reference id="ref.session">
   <title>Session Handling Functions</title>
   <titleabbrev>Sessions</titleabbrev>
@@ -44,6 +44,14 @@
      being not defined.  On subsequent accesses, these are not defined
      by the session module unless the user defines them later.
     </para>
+    <warning>
+     <para>
+      Some types of data can not be serialized thus stored in sessions. It
+      includes <type>resource</type> variables or objects with circular
+      references (i.e. objects which passes a reference to itself to another
+      object).
+     </para>
+    </warning>
     <note>
      <para>
       Session handling was added in PHP 4.0.