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XAMPP is intended for developer and testing purposes. To use it in a production site, you must harden it a lot for security. See the accompagning readme files.
In XAMPP I find it usual easier to install XOOPS in a subdirectory of /xampp/htdocs/
In that case you have to put in this directory a html or script to automatic transfert to the XOOPS directory.
For being called from the net, you must use as XOOPS_URL the URL or IP address that users use when browsing from the internet. An additional problem can be the firewall or router which must allow outbound connects via port 80. Be very careful or you will become very rapid hacked and/or your networked PC's content stolen.