php - ajax update textfield from database

I am new to ajax and i wanna ask a question that i tried to fix alot but couldn't figure out how. I have a html form which have 2 textfield areas. And i want to update second one from my database when user enter a value to first textfield.

I went through many online tutorials and i myself wrote something like this:

html:

<tr>
    <td width="20"></td>
    <td width="229" bgcolor="#FCECEC">ID</td>
    <td width="319" bgcolor="#FCECEC">NAME</td>  
    <td width="82" align="left"><label>
        <input type="button" name="ADD" id="ADD" value="New User" onclick=" "/>
    </label></td>
</tr>
<tr>  
    <td bgcolor="#FCECEC" align="center">1</td>
    <td><input type="text" name="ID" id="ID" maxlength="11" onchange="showUser(this.value)"   value=" " /></td>
    <td><input type="text" size="40" name="NAME" id="NAME" maxlength="80" value="" /></td>
</tr> 
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ajax function:

function showUser(str)
{
    if (str=="")
    {
        document.getElementById("NAME").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("NAME").innerHTML=xmlhttp.responseText;
        }
    }
    xmlhttp.open("GET","/myproject/users_update.php?q="+str,true);
    xmlhttp.send();
}
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and a part of php side:

$q=$_REQUEST["q"];
$kps->WhatsId($q);
$tname=$kps->NAME;
$tsname=$kps->SURNAME;
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When i do that i keep getting

undefined index q

Where did i go wrong? Is it any true? Any help would be appreciated. Thank you!