Checkbox related question with php and ajax

[alicefreak]

Hello guys, My question is when user click on checkbox, then user is activated. this code is working fine. But when user again click on uncheckbox, then again user de-activated. How to do that? here is my working code.

 

Activate user when user click on checkbo

if($_GET['doAction'] == 'Activate') {
    if(!empty($_GET['q'])) {
        $userid = $_GET['q'];
        $conn = db_connection();
        $query = "UPDATE user SET activate = '1' WHERE userid = '".$userid."' ";
        $result=$conn->query($query);
    }
}

Here is my checkbox button

<input type="checkbox" name="app" onchange="callUser(this.value,doAction.value);" value="<?php echo $userid;?>" <?php if($row['approved'] == '1'){ echo "checked=\"true\""; }?>/>
<input type="hidden" name="doAction" id="doAction" value="Approved" />

Here is callUser ajax function

<script type="text/javascript">
function callUser(str,action,third)
{
var xmlhttp;    
if (str=="")
{
document.getElementById("txtHint").innerHTML="";
return;
}
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari 
  xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
document.getElementById("txtHint").innerHTML=xmlhttp.responseText;

}
}
xmlhttp.open("GET","adminPanel.php?    q="+str+"&doAction="+action+"&app="+third,true);
    xmlhttp.send();
}
</script>

Thanks please help me

.  Here is screenshot http://i.imgur.com/MpQfC.png