Hi, i have this problem regarding ajax dropdownlist. Here is my code.

this is my ajax code
Code:
function getXMLHTTP() 
{ 
	var xmlhttp=false;	
	try
	{
		xmlhttp=new XMLHttpRequest();
	}
	catch(e)	
	{		
			
		try
		{			
			xmlhttp= new ActiveXObject("Microsoft.XMLHTTP");
		}
		catch(e)
		{
			try
			{
				xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
			}
			catch(e1)
			{
				xmlhttp=false;
			}
		}
	}
		 	
	return xmlhttp;
}
		
function getIpt(strURL) 
{		
		
	var req = getXMLHTTP();
		
	if (req) 
	{
			
		req.onreadystatechange = function() 
		{
			if (req.readyState == 4) 
			{
				// only if "OK"
				if (req.status == 200) 
				{						
					document.getElementById('institusidiv').innerHTML=req.responseText;						
				} 
				else 
				{
					alert("There was a problem while using XMLHTTP:\n" + req.statusText);
				}
			}				
		}			
		req.open("GET", strURL, true);
		req.send(null);
	}					
}

function getKursus(strURL) 
{		
		
	var req = getXMLHTTP();
		
	if (req) 
	{
			
		req.onreadystatechange = function() 
		{
			if (req.readyState == 4) 
			{
				// only if "OK"
				if (req.status == 200) 
				{						
					document.getElementById('kursusdiv').innerHTML=req.responseText;						
				} 
				else 
				{
					alert("There was a problem while using XMLHTTP:\n" + req.statusText);
				}
			}				
		}			
		req.open("GET", strURL, true);
		req.send(null);
	}					
}

Here is my php code (ajaxIPT.php)
Code:
<?php
require_once("../../class/class.mysqldb.php");
$code = intval($_REQUEST['code']);
$kod_ins = $_REQUEST['kod_ins'];
$con = new MysqlDB;
$con->connectMysql();


if($code == 1)
{
	$queryIpt = "select ctext from kod_ipt where kat_ipt = 1";
	$resultQuery = $con->query($queryIpt);
?>
	<select name="INSTITUSI" id="INSTITUSI">
	<option>--Choose--</option>
	<?php
		while($row=$con->fetch_array($resultQuery)) 
		{ 
	?>
	<option value="<?php echo $row['ctext']; ?>"><?php echo $row['ctext']; ?></option>
	<?php 
		}
	?>
	</select>
<?php 
}
elseif($code == 2)
{
	$queryIpt = "select ctext from kod_ipt where kat_ipt = 2";
	$resultQuery = $con->query($queryIpt);
?>
    <select name="INSTITUSI" id="INSTITUSI">
    <option>--Choose--</option>
    <?php
        while($row=$con->fetch_array($resultQuery)) 
        { 
    ?>
    <option value="<?php echo $row['ctext']; ?>"><?php echo $row['ctext']; ?></option>
    <?php 
        }
    ?>
    </select>
<?php 
}
elseif($code == 3)
{
?>
	<input name="INSTITUSI" type="text" id="INSTITUSI" size="30" value="" />
<?php 
}
elseif($code == 4)
{
?>
	<input name="INSTITUSI" type="text" id="INSTITUSI" size="30" value="Test" /> 
<?php
}
?>
And this is my form
Code:
<table width="100%" cellspacing="1" class="main">
<tr>
      <th width="25%">
      <div align="left">
      <strong>A</strong>
      </div>
      </th>
      <th align="center" width="2%">
      <div align="center">:</div>
      </th>
      <td width="73%" class="bodystyle">
            <select name="KOD_INSTITUSI" id="KOD_INSTITUSI" onchange="getIpt('ajaxIPT.php?code='+this.value),tukarPeringkat(this.value);">
            <option value="">-Choose-</option>
            <?php
                     while($row=$con->fetch_array($resultIptQuery)) 
                     { 
            ?>
                      <option value = "<?php echo $row['id']; ?>"><?php echo $row['cat']; ?></option>
            <?php 
                       }
            ?>
            </select> 
      </td>
</tr>
<tr>
      <th>
      <div align="left">
      <strong>B</strong>
      </div>
      </th>
      <th align="center"><div align="center">:</div></th>
      <td class="bodystyle">
      <div id="institusidiv">
            <select name="INSTITUSI" id="INSTITUSI" onchange="">
            <option value="">-Choose-</option>
            </select>
      </div>                    
      </td>
</tr>
</table>
this code works perfectly. When i choose KOD_INSTITUSI, the INSTITUSI dropdownlist change. The problem now is, when i post the form, the INSTITUSI IS NULL. It is like, the result in INSTITUSI dropdownlist only "virtually/temporarily"(sorry if im using wrong word ) exist there not the real object. That's y when i post the form, the INSTITUSI dropdownlist post NULL. I think maybe it is because the way i send data from ajaxIPT.php to the ajax code?? Hope someone out there can help me. Thanks in advance.