计算2000年01月01日起到指定日的天数

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