You can't just minus 7 from $file (which = "2003-02-20"). You have to break the date up into three parts... This would work...
$days = 7;
$year = date("Y");
$month = date("m");
$day = date("d");
$day_to_fetch = $day - $days;
include("data\".$year."-".$month."-".$day_to_fetch.".txt");
Now the problem is what if you are only on day 5 of Feb. and you want to go back 7 days? There is no -2 of Feb! You will have to make a couple of arrays... One with the name of each month with how many days in that month and one to accommodate for leap year with a year array. You can do the math from here. This is going to take you some time to put together...
Deo