php计算到指定日期还有多少天的方法，php日期

function countdays($d)
{
 $olddate = substr($d, 4);
 $newdate = date(Y) ."".$olddate;
 $nextyear = date(Y)+1 ."".$olddate;
  if($newdate > date("Y-m-d"))
  {
  $start_ts = strtotime($newdate);
  $end_ts = strtotime(date("Y-m-d"));
  $diff = $end_ts - $start_ts;
  $n = round($diff / 86400);
  $return = substr($n, 1);
  return $return;
  }
  else
  {
  $start_ts = strtotime($nextyear);
  $end_ts = strtotime(date("Y-m-d"));
  $diff = $end_ts - $start_ts;
  $n = round($diff / 86400);
  $return = substr($n, 1);
  return $return;
  }
}