http://opensourcecms.com/
This site allows you to do whatever you want with a CMS system for testing purposes. It gives you administrative access to the control panel so you can create/delete content without messing up something and to see if it is easy enough for you to use. The systems are wiped out after a predetermined time of 2 hours, so that everything is refreshed again.
In addition to CMS, there are blogging systems, forums, e-commerce, image galleries, e-learning, wikis and other PHP things to test out there. If you can't fin d it available, try looking in this list to see if it was one that was discontinued or that gave errors during installation.
Hope this helps and have fun!

