The topic of open source cms was discussed on the web standards group mailing list, so I thought I should check SS out since someone mentioned how good it was.
I would like to get into customising sites using SS. And I would also like to manipulate modules and CSS, would this be possible in SS.
Which explains the basic fundamentals of SilverStripe - where everything goes, an introduction to the CMS, and creation of your first basic template and how to manipulate it.
It also starts with some basic PHP - creating a new page type and manipulating it in SilverStripe - this is the first piece of PHP you'll be creating. In the succeeding tutorials you dive into it some more, creating a poll for starters, and a search form later on.
If you're looking to learn more advanced PHP, then it's probably a good idea to find some sort of tutorial on the web, or purchase a book.