PHP-Nuke is a Content Management System (CMS) which is written in PHP and uses a database, most commonly a MySQL database, but you can use a number of others such as mSQL, PostgreSQL, ODBC, ADABAS, Sybase or InterBase.
Version 7.5 and before are free to use as they are released under GNU General Public License as free software, after are 7.5 you have to pay!
There are many plugins to the software which will allow you to add new features to your website.