"simple" question

I've just done something similar myself, but i do have a php site with logon security already and that makes life easier. Firstly reorganise your site to put the text you don't mind the user changing into a separate file or files which can be 'included' back into their original page via php. Then, after successful logon, you can read the file data into the fckEditor pane ready for updating. When the user then clicks on a 'Ready' or similar button, 'Post' is used to transfer to a target php page which processes the text. In this page I displayed the changed text back to the user and added a confirm button only after which I carried out the update and re-presented the final page as it will look. This was in a new window so it was easy to go back to change the text if they wanted to.
In my case I have numerous blocks of text that the user can select to edit. I deal with this by keeping them all in a separate directory (e.g. includes/) and reading that directory and listing the files by name in a drop-down (select) box. In my case the names are meaningful, otherwise you might need a table to translate the filename into something understandable by the user.
I hope this helps.