Plone is the best open source CMS compliant with W3C's AA rating. There are many others but they charged you extra and have one commercial ( with more frill) and another one as open source.
But one thing is that it is not in PHP but python and server is also not Apache but Zope ( you can use Apache for caching). Both are mainly in python- one of the easiest language to learn. Also it supports 40 languages and is an easy to use platform for managing multi-language sites. Many books are also available for it.

Also training is provided by more than 300 organizations. It users include heavy weights like FSF , NASA and Amnesty International Switzerland.
It supports many major DBMS like oracle, mysql, postgresql, sqlite. It current version is 3.3.4.
Oh, you can still use it if you don't know much about python as PHP is supported indirectly and can be used.
PS: I don't meant to say that Joomla or wordpress are bad but they are not enterprise CMS ( which usually cost 1000$ per CPU) as they are not international standard complaints. They are good for other uses.