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Ok.... Here's how you do it.
Say you have /home/mywebsite/public_html/
Upload your XOOPS installation there and install it, etc.
Make a copy of your mainfile.php and rename it to ABC.mainfile.php
Edit mainfile.php to have the modified code.
Point
www.ABC.com to /home/mywebsite/public_html/
Basically when you goto
www.ABC.com it will first goto read mainfile.php but mainfile.php has code in it to grab the domain name ABC and then direct it to ABC.mainfile.php
Next make a copy of the database and the ABC.mainfile.php but call it DEF.mainfile.php
Edit DEF.mainfile.php with the new database information.
Point
www.DEF.com to /home/mywebsite/public_html/
Now when you goto
www.DEF.com it will grab the information from DEF.mainfile.php
For every other site you want running of the same install just make a new copy of the database and a new mainfile.php to match it.