Sounds like the thing is written for a really old version of PHP.
Try asking on their forums about whether there is a newer version of dokeos, or which version of PHP the current version written for so that you can downgrade PHP.
Failing that, turn register_globals off (it has nothing to do with this, it's a bad idea, and has itself been deprecated for some years); to turn off Deprecated: warnings have an error_reporting setting in php.ini like [font=monospace]error_reporting: E_ALL & ~E_DEPRECATED[/font] (for development and testing) and [font=monospace]error_reporting: E_NONE[/font] (for real life use).
But that will only be a short-term solution. PHP 5.4 will probably be released before the end of the year, and it doesn't even have [man]session_unregister[/man]; you won't be able to upgrade as long as this software uses it.