TP 查询某一条件字段 并 排序

例：

SELECT `content` FROM `app_sms_statistics` WHERE ( mobile= "15300330047" ) ORDER BY time DESC LIMIT 1

$sms_statistics =  M("sms_statistics");   
$sms_statis = $sms_statistics->order("time DESC")->where("mobile= ".""$phone"")->getField("content");
echo $sms_statistics->getlastsql();