变量无法赋值是怎么回事?$Starttime=$_get["starttime"];$Endtime=$_get["endtime"];$Time_temp="and createdate>='$Starttime' and createdate<='$Endtime'";$starttime,$endtime都有值,请问echo $time_temp输入为:and createdate>='' and createdate<=''是怎么回事?