Some data can be serialized (bug #17419)

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@168962 c90b9560-bf6c-de11-be94-00142212c4b1

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.