Added description of the problem

Author: hmic

en/development/configuration.rst

@@ -671,6 +671,9 @@ In your ``app/Config/bootstrap.php`` you could attach this reader and use it::
  Configure::config('xml', new MyXmlReader());
  ...
 
+It is not a good idea to call your custom configure class ``XmlReader`` because that classname is an internal PHP one already:
+http://php.net/manual/de/book.xmlreader.php
+
  Configure::load('my_xml');
 
 The ``read()`` method of a config reader, must return an array of the configuration information