为啥不能调用函数里面的变量

为什么不能调用函数里面的变量？

//定义常量

define("EntTime", "2012-08-01");

define("EntTime2", "2012-08-31");

define("Query_field", "品号");

define("Operate", "包含");

define("requirement", "WDZ");



//将常量转换为变量

$EntTime = EntTime;

$EntTime2 = EntTime2;

$Query_field = Query_field;

$Operate = Operate;

$requirement = requirement;



//自定义函数

function jhRepPd(){

	GLOBAL $PUR,$MOC;

	switch($Operate){

		case "包含":

			if($Query_field=="品号"){

				$PUR = "PURTH.TH004 like'%".$requirement."%' AND ";

			}

			break;

	}

}



//去除日期中的"-"

$a_date = "PURTG.TG003 >='".str_replace("-","",$EntTime)."'";

$b_date = "PURTG.TG003 <='".str_replace("-","",$EntTime2)."'";



//判断变量是否为空

if(!empty($EntTime) && !empty($EntTime2) && $requirement!==""){

	$date = "(".$a_date." AND ".$b_date.") AND ";

	jhRepPd();

};



//sql语句

$sql = "SELECT * FROM TB where {$date}{$PUR}dbId in('1','2','3')";



//打印SQL语句

echo $sql;



?>

--这是打印结果，但不是正确的。因为函数中的变量没有

输出，为什么？
SELECT * FROM TB where (PURTG.TG003 >='20120801' AND PURTG.TG003 <='20120831') AND dbId in('1','2','3')

--正确的结果应该是：
SELECT * FROM TB where (PURTG.TG003 >='20120801' AND PURTG.TG003 <='20120831') AND PURTH.TH004 like'%WDZ%' AND dbId in('1','2','3')

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------解决方案--------------------
function jhRepPd(){
GLOBAL $PUR,$MOC;
switch($Operate){
case "包含":
if($Query_field=="品号"){
$PUR = "PURTH.TH004 like'%".$requirement."%' AND ";
}