php计算登陆时间差

/*
			处理显示时间
			@param   	$time datetime类型
			@return 	str
		*/
		private function _doTime($time) {
			if(empty($time))
				return;

			//转为时间搓
			$time_diff 				= time() - strtotime($time);
			$re_str 				= '';

			switch ($time_diff) {
				case $time_diff <= 60:
					$re_str 		= floor($time_diff) . '秒之前';
					break;
				case $time_diff <= 3600:
					$re_str 		= floor($time_diff / 60) . '分之前';
					break;
				case $time_diff <= 3600 * 24:
					$re_str 		= floor($time_diff / 3600) . '小时之前';
					break;
				case $time_diff <= 3600 * 24 * 30:
					$re_str 		= floor($time_diff / 3600 / 24 ) . '天之前';
					break;
				case $time_diff <= 3600 * 24 * 30 * 12:
					$re_str 		= floor($time_diff / 3600 / 24 / 30 ) . '个月以前';
					break;
				default:
					$re_str 		= floor($time_diff / 3600 / 24 / 30 / 12) . '年以前';
					break;
			}

			return $re_str;
		}