ckfinder change basepath on the fly

You change the baseUrl and baseDir in the config.php file of CKFinder.

if user=a then use these settings, else use these other settings.

But you must do it in the config.php file.

Why isn't that a good solution?

You need to specify the server paths only at a server script. If you allow to specify them by a URL parameter you are running the risk that someone will be able to hack your server specifying their own paths.

Remember also that the config.php file can include other scripts to get extra info from your CMS, anything that you can do with another file manager in this regard is also doable with CKFinder.

It's pretty easy to do. I add the user id to the base path to give each use their own private directory.