are you trying to create something very unique compared to the already existing ones?
Joomla is one of the famous open source CMS, i would suggest reading document section of joomla to get more ideas http://docs.joomla.org/
why dont you consider few CMS system out there, see whether you can find something suite for your requirements. Probably you may able to create some plugins or modification to existing CMS systems to make it work for your projects.
Then (re)use an existing cms if I were you, unless you like learning.
Go to opensourcecms.com click on PHP, click on "lite" (LH menu) find a couple that fit your specification, check they are still maintained (if they are hosted on sourceforge.net this can be checked), check they are PHP5 only (if you are on PHP5) check the licences, download them, have a play.