Mysql 带有日期时间的查询问题和日期之间的查询

我有一个疑问：

$result = "SELECT forma.*, SMS_MONTIME.IDTICKET, SMS_MONTIME.MBYLLUR,SMS_MONTIME.time_added
FROM forma 
LEFT JOIN SMS_MONTIME ON forma.ID = SMS_MONTIME.IDTICKET WHERE forma.data_fillim >= '".$df."' AND forma.data_fillim <= '".$dm."' ORDER BY forma.id DESC";  

如果我这样说的话，查询工作正常：

$result = "SELECT forma.*, SMS_MONTIME.IDTICKET, SMS_MONTIME.MBYLLUR,SMS_MONTIME.time_added
FROM forma 
LEFT JOIN SMS_MONTIME ON forma.ID = SMS_MONTIME.IDTICKET WHERE forma.data_fillim >= '20130621' AND forma.data_fillim <= '20130621' ORDER BY forma.id DESC";

$result=“选择格式。*、SMS\u MONTIME.IDTICKET、SMS\u MONTIME.MBYLLUR、SMS\u MONTIME.time\u已添加
从形式
左键连接forma.ID=SMS\u MONTIME.IDTICKET上的SMS\u MONTIME，其中forma.data\u fillim>='20130621'和forma.data\u fillim在连接SQL字符串时是否尝试将$dm和$df转换为字符串（）；数据填充的数据类型是什么
$result = "SELECT forma.*, SMS_MONTIME.IDTICKET, SMS_MONTIME.MBYLLUR,SMS_MONTIME.time_added
FROM forma 
LEFT JOIN SMS_MONTIME ON forma.ID = SMS_MONTIME.IDTICKET WHERE forma.data_fillim >= '20130621' AND forma.data_fillim <= '20130621' ORDER BY forma.id DESC";