Checkbox related question with php and ajax

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