PHP计算当前日期几天或后几天日期的函数

public static function subDate($day, $date = "", $timestampFlag = true, $format = "Y-m-d H:i:s") {
if (empty($date)) {
$now = time();
} else {
注释: 不是时间截格式，是YYYY-mm-dd HH:ii:ss格式，将它们转换成时间戳格式
if (!is_numeric($date)) {
$datetime_array = explode(" ", $date);
$date_array = explode("-", $datetime_array[0]);
$time_array = explode(":", $datetime_array[1]);
$now = mktime($time_array[0], $time_array[1], $time_array[2], $date_array[1], $date_array[2], $date_array[0]);
} else {
$now = $date;
}
}
$m = $day * 24 * 60 * 60;
$diff = $now + $m;
if ($timestampFlag) {
return $diff;
} else {
if (empty($format)) $format = "Y-m-d H:i:s";
$oldday = date($format, $diff);
return $oldday;
}
}