SilverStripe is a free open source content management system that is coded in PHP and supports MySQL, Microsoft SQL Server and Postgres as database backends. It contains the Sapphire model-view-controller framework and is extremely extendable. There are a number of modules, themes and widgets abailable and it is easily customisable. The admin interface is slick and can also be customised with additional tabs and fields depending on the page types.
Be sure to check out the online documentation and SSBits.com. The latter is full of all sorts of useful tips and tricks and is worth spending time looking through after reading the docs.
The default .htaccess file for SilverStripe runs all URLs that do not belong to actual files on the filesystem through the Sapphire framework. This means that if a request is made for a CSS file that does not exist, for example, it will be run through SilverStripe/Sapphire, which is not really necessary.