diff --git a/appendices/migration51.xml b/appendices/migration51.xml
index e58df738b5..6ea71cdc3b 100644
--- a/appendices/migration51.xml
+++ b/appendices/migration51.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="iso-8859-1"?>
-<!-- $Revision: 1.4 $ -->
+<!-- $Revision: 1.5 $ -->
 <appendix xml:id="migration51" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
  <title>Migrating from PHP 5.0.x to PHP 5.1.x</title>
 
@@ -244,7 +244,7 @@ var_dump($bar);
     </programlisting>
    </informalexample>
    <para>
-    In PHP 5.0.3, <varname>$bar</varname> evaluated to <literal>NULL</literal>
+    In PHP 5.0.3, <varname>$bar</varname> evaluated to <constant>NULL</constant>
     instead of returning an object. That happened because
     <literal>getThis()</literal> returns by value, but the value here is
     assigned by reference. Although it now works in the expected way, this is