To try and help a little more, try doing this. . .
First off, if you're using a router, go into the router's configuration menu. This is commonly located by going to 192.168.1.1 in your browser, however it my be different depending on your router's defaults. Check your router manual for how to access it.
Now look around until you find the DHCP Client table for your LAN (Usually this is found under Status). You should see your server's computer name listed and the local IP for it, such as 192.168.1.111 or something.
Now look around again until you find Port Forwarding. Enter any name for the application and then forward PORT 80. Usually there's two places to enter the port number, just enter 80 both times. Then under the IP, enter your server's IP and make sure this entry is enabled. Save it and then close it out. Now if all goes well, you should be able to reach your server via your outside IP address.
** Note: For some functions to work properly, you will have to forward additional ports to your server. For example, if you want to use FTP, you must forward Port 21 to your server. (assuming you have an FTP Server Software running on the Server PC). There should be a list somewhere of all the ports that need to be enabled for XAMPP.
I have a Video Tutorial I made for Windows XAMPP, but the configurations should be very similar for Apache on Linux minus the directory settings. You can check it out here: http://www.imgoingtoreportyou.com/phpBB3/viewtopic.php?f=12&t=27
. There's also a tutorial in that same section on how to direct a Domain Name to your IP address if you decide you'd like to buy a domain name and use it instead. Feel free to leave comments there and I will assist you the best I can.
I'm currently configuring a XAMPP server in Linux, so I will make some Linux Version tutorials as soon as I get it properly working and figured out.
I hope this helps ya.