若要使用PHP来比较日期，最好用DateTime::diff

但是这个是5.3才支持的，如果没有这样的环境，可以使用<、>来比较

如下例子，会输出right

$date1=strtotime('2009-8-9 20:00');
$date2=strtotime('2009-9-9 1:00');
if($date1<$date2)
echo 'right';
else
echo 'wrong';

注：DateTime::diff

DateTime::diff

(PHP 5 >= 5.3.0)

DateTime::diff — Returns the difference between two DateTime objects

Description

public DateInterval DateTime::diff ( DateTime $datetime [, bool $absolute ] )

Returns the difference between two DateTime objects.

Parameters

datetime

The date to compare to.

absolute

Whether to return absolute difference. Defaults to FALSE.

Return Values

The difference between two dates.