diff --git a/reference/session/functions/session-name.xml b/reference/session/functions/session-name.xml
index 151f57f930..fe37a0868a 100644
--- a/reference/session/functions/session-name.xml
+++ b/reference/session/functions/session-name.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.4 $ -->
+<!-- $Revision: 1.5 $ -->
 <!-- splitted from ./en/functions/session.xml, last change in rev 1.2 -->
   <refentry id="function.session-name">
    <refnamediv>
@@ -27,6 +27,12 @@
      for every request (and before <function>session_start</function>
      or <function>session_register</function> are called).
     </para>
+    <warning>
+     <para>
+      Session name can't consist only from digits, at least one letter must be
+      present. Otherwise new session id is generated every time.
+     </para>
+    </warning>
     <example>
      <title><function>session_name</function> examples</title>
      <programlisting role="php">