<p>I want to write a little app for myself which notifies me if there is a new version of php or mysql but I would need a feed or an xml file to read it from.</p>

PHP.net provides one
http://www.php.net/releases/index.php?serialize=1
An alternative that may allow you to add additional software later is looking into the feeds provided by freshmeat:
http://freshmeat.net/projects/mysql/