there isn't a XAMPP 'config' file really. XAMPP/LAMPP is a bash file/script, ratehr than an installed LMAP stack. The lampp script (executable) is located in /opt/lampp. You should open it in a programming editor (like geany) where the syntax is parsed and highlighted so it is more understandable, it will be very enlightening.
If you are asking about the Apache or MySQL config files, read here (bottom of page):http://www.apachefriends.org/en/xampp-linux.html