PHP插入MSSQL数据问题

[yuzhifu1]

<?php
$serverName = "192.168.0.2";
$connectionInfo = array( "UID"=>"SA", "PWD"=>"GUOOUG", "Database"=>"AIS20131025233823");
$conn = sqlsrv_connect( $serverName, $connectionInfo);

include_once 'inc/auth.inc.php';
include_once("../../plugin.inc.php");
ob_end_clean();
$RUN_DATA = getrundata($RUN_ID);
$USER_ID = $RUN_DATA['车间'];//车间
$AGENT = $RUN_DATA['币别'];//币别

$sql = "insert into A_4(a1,a2) values('$USER_ID','$AGENT')";
$stmt = sqlsrv_query( $conn, $sql );  

if (!$stmt){
echo "<script language=\"JavaScript\">alert(\"插入单据号失败，请检查！\");</script>";
 }
?>

在PHP中插入到MSSQL，程序没执行，请大神帮忙看下哪里写得不对，谢谢！

[toorup]

values里的值是变量，为什么要引号引起来？
另echo一下这两个变量有值没有

[toorup]

$sql = "insert into A_4(a1,a2) values('".$USER_ID."','".$AGENT."')";