php计算两个日期相差的天数

/**
 * 时间差计算
 *
 * @param Timestamp $time
 * @return String Time Elapsed
 */
function time2Units ($time,$sign)
{
  $year   = floor($time / 60 / 60 / 24 / 365);
  $time  -= $year * 60 * 60 * 24 * 365;
  $month  = floor($time / 60 / 60 / 24 / 30);
  $time  -= $month * 60 * 60 * 24 * 30;
  $week   = floor($time / 60 / 60 / 24 / 7);
  $time  -= $week * 60 * 60 * 24 * 7;
  $day    = floor($time / 60 / 60 / 24);
  $time  -= $day * 60 * 60 * 24;
  $hour   = floor($time / 60 / 60);
  $time  -= $hour * 60 * 60;
  $minute = floor($time / 60);
  $time  -= $minute * 60;
  $second = $time;
  $elapse = '';
  $unitArr = array(
        '年'  =>'year',
        '个月'=>'month',
        '周'=>'week',
        '天'=>'day',
        '小时'=>'hour',
        '分钟'=>'minute',
        '秒'=>'second'
    );
   foreach ( $unitArr as $cn => $u )
   {

       if ( $$u > 0 )
       {
          if($sign == 'day'){
            $elapse = $$u . $cn;
            break;
          }else{
            //上两行只取最大时间（有分钟就舍去秒，有小时就舍去分秒，以此类推）
            $elapse .= $$u . $cn;
          }
       }
   }
   return $elapse;
}

其中的$$u表示将$u作为名称，重新声明变量

假如$u的值为year，则$$u相当于$year,即输出$year的值