PHP计算时间差，并返回什么时间之前发表的内容

<?php
header("Content-type: text/html; charset=utf-8");
function time2Units ($time){
$year = floor($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 ( $$u > 0 ){
$elapse = $$u . $cn;
break;
}
}
return $elapse;
}
$past = strtotime("2015-01-12 21:49:00"); // Some timestamp in the past
$now = time(); // Current timestamp
$diff = $now - $past;
echo '发表于' . time2Units($diff) . '前';

?>