diff --git a/language/namespaces.xml b/language/namespaces.xml
index 15efffbf21..96b043bf66 100644
--- a/language/namespaces.xml
+++ b/language/namespaces.xml
@@ -69,6 +69,13 @@ echo constant($d); // see "Namespaces and dynamic language features" section
     ]]>
    </programlisting>
   </example>
+  <note>
+   <para>
+    Namespace names <literal>PHP</literal> and <literal>php</literal>, and compound names starting
+    with these names (like <literal>PHP\Classes</literal>) are reserved for internal language use 
+    and should not be used in the userspace code. 
+   </para>
+  </note>
  </sect1>
 
  <sect1 xml:id="language.namespaces.definition">