I take it this is xampp for Linux (as chmod/chown would be nonsensical on windows)
To find the user just issue, as root,
ps -aux | grep apache
Left hand column is user, you may have a couple running as root, but the majority will usually be running as 'nobody' or 'www-data'.
to change the user and group at the same time - you'll probably want to do the lot so:
chown -R www-data:www-data /path/to/directory/*
Or safer to do the directory itself, then cd in and issue:
chown -R www-data:www-data *