Calculates the number of days from January 01, 2000 to a specified day _php basis

This is a function that calculates the number of days from January 01, 2000 to a specified day
<br>
(Calculate head also count tail) date format is: YYYY-MM-DD
<br>

<?php
Calculate the number of days from January 01, 2000 to one day

function Cal_start2end ($end _day, $start _day)
{


$start _day=ereg_replace ("-", "", $start _day);
$end _day=ereg_replace ("-", "", $end _day);



if ($end _day>= $start _day)
{//Due date is greater than start date
if (substr ($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;


Same year

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
{
Different years!

$differ _year=substr ($end _day,0,4)-substr ($start _day,0,4);

$how _int_4_floor=floor ($differ _year/4) +1; Rounding down
$how _int_4_ceil=ceil ($differ _year/4) +1; Rounding up



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 "You entered is:". $end _day. " <br>\n ";
$endday _from_startday=cal_start2end ($end _day, $start _day);

echo "This day distance $start_day altogether". $endday _from_startday. "Day";

?>
<br><br><br><br> time is too little, the prawn can be modified to calculate the interval between any two dates best!!!!!