How to self-update PHP+MySQL CMS?

I'm writing a CMS on PHP+MySQL. I want it to be self-updatable (throw one click in admin panel). What are the best practices?
How to compare current version of cms and a version of the update (application itself and database). Should it just download zip archive, upzip it and overwrite files? (but what to do with files that are no longer used). How to check if an update is downloaded correctly? Also it supports modules and I want this modules to be downloadable from the admin panel of cms.
And how should I update MySQL tables?

A slightly more experimental solution could be to use something like the phpsvnclient library.

With features:

• List all files in a given SVN repository directory
• Retrieve a given revision of a file
• Retrieve the log of changes made in a repository or in a given file between two revisions
• Get the repository latest revision

This way you can see if there are new files, removed files or updated files and only change those in your local application.

I recon this will be a little harder to implement, but the benefit would probably be that it is easier and quicker to add updates to your CMS.