PHP function to calculate the number of days difference between two dates

/*
*
*Function function: Calculate the difference of two dates in YYYY-MM-DD format by how many days
*
*/
function getChaBetweenTwoDate($date1,$date2 ){
$Date_List_a1=explode("-",$date1);
$Date_List_a2=explode("-",$date2);
$d1=mktime(0,0,0,$Date_List_a1[1 ],$Date_List_a1[2],$Date_List_a1[0]);
$d2=mktime(0,0,0,$Date_List_a2[1],$Date_List_a2[2],$Date_List_a2[0]);
$Days= round(($d1-$d2)/3600/24);
return $Days;
}
echo getChaBetweenTwoDate('2010-08-11','2010-08-16' );
echo "";
echo getChaBetweenTwoDate('2010-08-16','2010-08-11');
?>