> $today = date("Ymd", mktime(0,0,0, date(m),date(d),date(Y)));
> $last_week = date("Ymd", mktime(0,0,0, date(m),date(d)-7,date(Y)));
> echo ($today - $last_week);
> The result is a number like 8876 (20020107-20011231 = 8876)
> But in date thinking it should be 7!
No, that's the difference in time represented by the number of seconds.
You still need to work with it a little more.
8876 / 60 = number of hours /* 60 = number of seconds in an hour */
8876 / 60 / 24 = number of days. /* 24 = number of hours in a day */