在线等，关于上传文件

谁能帮我dubug这个程序：
index.php是上传表单：
<?php$filename=$_GET['filename'];
echo $filename;
echo "<script language=\"JavaScript\" type=\"text/JavaScript\"> var filename='".$filename."';</script>";
?>
<body><form method="post" action="upload.php" enctype="multipart/form-data" target="up_frm">
<input type="button" value="Compare" class="btn" onClick="return submitComp()"/>
<br><b>Or upload file:<br></b>
<input type="file" name="upload" id="upload" style="font-family:arial;width:271px;height:20px">
<input type="submit"  id="filesub"  style="visibility:hidden"/>
</form><iframe name=up_frm style="display:none">
</iframe></body>
</html><script type="text/javascript">
alert("step1"+filename);
function submitComp(){
alert("init");
document.getElementById("filesub").click(); 
alert("step2"+filename);
}

</script>
upload.php代码如下：<?php$filename="NULL";
if ($_FILES["upload"]["error"] > 0){
$filename="Error: ".$_FILES["upload"]["error"];
}else{
if(is_uploaded_file($_FILES["upload"]["tmp_name"])){
$arrname=explode(".",strval($_FILES["upload"]["name"]));
$filename=array_pop($arrname);
$filename=array_pop($arrname).".".$filename;
move_uploaded_file($_FILES["upload"]["tmp_name"],"./temp/upload/".$filename);
}else{
$filename='FLASE';
}
}
header('Location:index.php?filename='.$filename);
?>可是在index.php里，收到upload.php传回的值后，在js代码里，获得的filename始终不对。
请达人指教！