php time and date calculation

//时间计算 开始，结束，类型
function shijianjisuan($kaishi,$jieshu,$leixing='shi') {
    if ($leixing=='shi')//小时
    {
    $sc = floor((strtotime($jieshu) - strtotime($kaishi)) / 3600);
    }
    elseif ($leixing=='fen')//分钟
    {
    $sc = floor((strtotime($jieshu) - strtotime($kaishi)) / 60);
    }
    elseif ($leixing=='tian')//天
    {
       $sc = floor((strtotime($jieshu) - strtotime($kaishi)) / 3600/24); 
    }
    return $sc;
}
 
$a='2016-08-02';
$b='2016-08-04';
echo shijianjisuan($a,$b,'tian');