php实现时间显示方式（2秒前）

header("Content-type:text/html;charset=utf-8"); 

function relative_date($date) {
    if (!preg_match('/^\d+$/', $date)) $date = strtotime(trim($date));
    $sec = time() - $date;
    switch(true){
    	case $sec < 60:
    		return 60 - $sec. '秒前';
        case $sec < 3600:
            return round($sec/60). '分钟前';
        case $sec < 86400:
            return round($sec/3600). '小时前';
        case $sec < (86400 * 7):
            return round($sec/86400). '天前';
        case $sec < (86400 * 7 * 4):
            return round($sec/(86400*7)). '周前'; 
        default:
            return date("Y年n月d日",$date);
    }
}
$date = '2014-11-20 13:36:12';
echo relative_date($date);