xmlHttp = new XMLHttpRequest();
function callServer(){

var id = document.getElementById('id').value;

var server = "./test2.php?id="+id;

xmlHttp.open("GET", server, true);

xmlHttp.onreadystatechange = updatePage;

xmlHttp.send(null);
}

function updatePage() {
  if (xmlHttp.readyState == 4) {
    var response = xmlHttp.responseText;
    alert(response);
    document.getElementById("url").value = response;
  }
}

<form>
<p>Id:<input type="text" id="id" onchange="callServer()"></p>
<p>URL:<input type="text" id="url"></p>
</form>

if(!empty($_GET['id'])){
$id = $_GET['id'];
$query = "SELECT * FROM `com` WHERE `id`=".$id;
$msq = mysql_query($query);

$arr = mysql_fetch_assoc($msq);

$response = $arr['url'];
return string $response;
}

if(!empty($_GET['id'])){
$id = $_GET['id'];
$query = "SELECT * FROM `com` WHERE `id`=".$id;
$msq = mysql_query($query);

$arr = mysql_fetch_assoc($msq);

$response = $arr['url'];
echo $response;
}