I'm sure you can do this with some math and a bit of careful thought. You need to check what day of the week the item was created and what day of the week it currently is. First, you calculate the number of days old it is, as I'm sure you were doing at first.
// if today is saturday, subtract 1 from the day. if its sunday, subtract 2
if (now() is sunday) $now = now() - 2 days;
if (now() is saturday) $now = now() - 1 day;
// same thing for $date posted. but adding 1 or 2 days
if ( $date is saturday) $date = $date + 2;
if ( $date is sunday) $date = $date + 1;
// get days difference
$days = $today - $date;
// have to subtract 2 days for every 7
$days = $days - floor($days/7)*2
You'd have to check if that works. Maybe you can do the calculation without moving your date to before/after the weekend. It may not be perfect, but its the right idea. No need to iterate.