diff --git a/reference/session/functions/session-id.xml b/reference/session/functions/session-id.xml
index 2d27121c5f..fdadd2d006 100644
--- a/reference/session/functions/session-id.xml
+++ b/reference/session/functions/session-id.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.2 $ -->
+<!-- $Revision: 1.3 $ -->
 <!-- splitted from ./en/functions/session.xml, last change in rev 1.2 -->
   <refentry id="function.session-id">
    <refnamediv>
@@ -14,13 +14,25 @@
      </methodsynopsis>
     <para>
      <function>session_id</function> returns the session id for the
-     current session. If <parameter>id</parameter> is specified, it
-     will replace the current session id.
+     current session.
+    </para>
+    <para>
+     If <parameter>id</parameter> is specified, it will replace the current
+     session id. <function>session_id</function> needs to be called before
+     <function>session_start</function> for that purpose. Depending on the
+     session handler, not all characters are allowed within the session id.
+     For example, the file session handler only allows characters in the range
+     <literal>a-z, A-Z and 0-9</literal>!
     </para>
     <para>
      The constant <systemitem>SID</systemitem> can also be used to
      retrieve the current name and session id as a string suitable for
-     adding to URLs.
+     adding to URLs. Note that <systemitem>SID</systemitem> is only defined if
+     the client didn't sent the right cookie. See also <link
+     linkend="ref.session">Session handling</link>.
+    </para>
+    <para>
+     See also <function>session_start</function>.
     </para>
    </refsect1>
   </refentry>