Sorry for crossposting, but I didn't find another place to make feature requests and I am afraid that the developers don't have time to look deep inside other threads.
If PHP was previously installed on the machine, it's likely to have set the environment variable PHPRC. This makes the "php.ini" file from the previous installation to be used instead of the XAMPP one.
This may lead to confusion. The worst case is when PHP is uninstalled but the environment variable remains. Then default settings are used and things don't work.
Preventing this problem is easy. Just change the default file "httpd-xampp.conf" and include the line "PHPIniDir <xampp install dir>\apache\bin\php.ini" before the "LoadModule php5_module". I believe this will lead to more expected behaviour.