这是一个计算2000年01月01日起到指定日的天数的函数
<br>
(算头也算尾)日期格式为：YYYY-MM-DD
<br>

<?php
//    计算从2000年01月01日开始到某日的天数

function cal_start2end($end_day,$start_day)
{


$start_day=ereg_replace("-","",$start_day);
$end_day=ereg_replace("-","",$end_day);



if($end_day>=$start_day)
{    //    截止日期大于开始日期
    if(substr($end_day,0,4)==substr($start_day,0,4))
        {

if( is_int(substr($end_day,0,4)/4) )
    $leap_day=29;    //leap year
else
    $leap_day=28;


//            //同一年

        if(substr($end_day,4,2)==substr($start_day,4,2))
            {
//            the same month  
            $endday_from_startday=$end_day-$start_day+1;

            }
        else
            {
//             not same month


            switch (substr($end_day,4,2))
                {
                case "01":
                        $endday_from_newyear=substr($end_day,6,2)+1;
                        break;
                case "02":
                        $endday_from_newyear=substr($end_day,6,2)+31;
                        break;
                case "03":
                        $endday_from_newyear=substr($end_day,6,2)+$leap_day+31;
                        break;
                case "04":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2;
                        break;
                case "05":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2+30;
                        break;
                case "06":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*3+30;
                        break;
                case "07":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2+30*2;
                        break;
                case "08":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*3+30*2;
                        break;
                case "09":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*2;
                        break;
                case "10":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*3;
                        break;
                case "11":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*4;
                        break;
                case "12":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*5;
                        break;
                }
            $endday_from_startday=$endday_from_newyear;
            }

return ($endday_from_startday);

        }
    else
        {
//        不同年！

$differ_year=substr($end_day,0,4)-substr($start_day,0,4);

$how_int_4_floor=floor($differ_year/4)+1; // 向下取整
$how_int_4_ceil=ceil($differ_year/4)+1; // 向上取整



if($how_int_4_floor==$how_int_4_ceil)
    $how_leap_year=$how_int_4_floor-1;
else
    $how_leap_year=$how_int_4_floor;

$how_noleap_year=$differ_year-$how_leap_year;

$differ_year2day=$how_noleap_year*365+$how_leap_year*366;



if( is_int(substr($end_day,0,4)/4) )
    $leap_day=29;    //leap year
else
    $leap_day=28;



        if(substr($end_day,4,2)==substr($start_day,4,2))
            {
//            the same month  
            $endday_from_startday=$differ_year2day+ substr($end_day,4,2);

            }
        else
            {
//             not same month


            switch (substr($end_day,4,2))
                {
                case "01":
                        $endday_from_newyear=substr($end_day,6,2)+1;
                        break;
                case "02":
                        $endday_from_newyear=substr($end_day,6,2)+1+31;
                        break;
                case "03":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31;
                        break;
                case "04":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2;
                        break;
                case "05":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2+30;
                        break;
                case "06":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*3+30;
                        break;
                case "07":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*2+30*2;
                        break;
                case "08":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*3+30*2;
                        break;
                case "09":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*2;
                        break;
                case "10":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*3;
                        break;
                case "11":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*4;
                        break;
                case "12":
                        $endday_from_newyear=substr($end_day,6,2)+1+$leap_day+31*5+30*5;
                        break;
                } //End of switch


    $endday_from_startday=$endday_from_newyear+$differ_year2day;
    }    //End of  not same month  



return ($endday_from_startday);


        } //End of year
}        //End of  $end_day>=$start_day



} //End of function


$start_day="2000-01-01";

$end_day="2011-01-01";

echo "您输入的是：".$end_day."<br>\n";
$endday_from_startday=cal_start2end($end_day,$start_day);

echo "该日距离$start_day 共 ".$endday_from_startday." 天";

?>
<br><br><br><br>时间太少了，那位大虾可以修改为计算任意两日期之间的间隔最好！！！！！