You might take a look at
http://midgard-project.org/
At least starting with an existing framework you would have a slight advantage. Since you chose PHP, you could use one of their CMS backends and stay with PHP code on top of their framework. Its not extremely lightweight, but, it does pretty well under load.
Otherwise, I think you're going to want to try using an existing CMS that has the modules you need -- perhaps start the search at
http://cmsmatrix.org/ and put in the options and see if you can find something that is close.