you'll need a database that associates each client with their list of viewable videos. you can use xml for that database or even maintain the database within your flash app but there several reasons to keep the database outside flash. and, you may want to a database that allows use of a query language.
you can use a query string appended to each client's link to your app but you may want to consider having clients log-in to access their videos especially if there's a good reason clients should not see particular videos.
Any samples of this?
Yeah its not really the kind of thing where you're gonna find a downloadable example. If you want help I could do it but it's going to take enough time that 10 points on a forum isn't gonna be worth it! lol. I can help if you want though PM me. Or, you if you've got time just start googling. You'll need:
A flash app that can post an XML playlist to PHP
a PHP script to catch XML and a user ID and write them to a database
a mysql database with a users table and a videos table. You'll either need to store the videos in the database as BLOB files or store URLS to the videos hidden somewhere in a folder that is world only readable
a php script that will accept a username and password, do a lookup of the user and return the XML map of availble videos they can watch
another php script that will accept a username and password and a video name, authenticate the request, lookup the file in the database or a reference to the file from the database and return the file in a stream back to flash.
and of course lastly you'll need the actual video player which will get ask the user to log in or get their credentials elsewhere and make requests to the above two php scripts.