Woo! An easy one! Try this first:
<?php
include ("$area.txt");
?>
If that doesn't work, then your server probably has register_globals=off. If so, try this:
<?php
inlclude ("$HTTP_GET_VARS[area].txt");
?>
Basically, if register_globals is off, then the vars in the URL don't get created as variables in the PHP page automatically so you have to pull them out of the array generated by the web server yourself.
Hopefullly that helps.
Adam