If you are constantly getting the errormessage: 

Fatal error: Unknown function: radius_auth_open() in...

And your Server is a Windows-System (for example standard-xampp installation), you propably did not remove the comment symbol ";" in front of "extension=php_radius.dll" in php.ini.

If you did that, but get the error anyway:

Additionally be sure you edited the right php.ini, since xampp installs several php.exe's but only "xampp/apache/bin/php.ini"  is the correct one!

It did cost me 2 days to find that out!