<? $host = 'localhost';
   $user = 'root';
   $pass = 'abcde';
   $db = 'register';
?>

<?
require ('config.php');

$link = mysqli_connect ($host,$user,$pass,$db);
$usernameq = "select user_name from register where username = '".$_POST['userN']."'";
$result = myqli_query($link,$usernameq)
if(!$result){
    die('invalid query');
    }
else {}
}
?>

<?
require ('dbquery.php');
?>

<script>
function matchWord(e){
    if(document.getElementById('userN').value.trim() === '<? '".$GET['username'].'"; ?>){
        document.getElementById('Upass').disabled = true;
        alert('User already exist choose another');
        }
    else{
        document.getElementById('Upass').disabled = false;
        }
    }
document.getElementById('userN').addEventListener("keyup", function(evt) {
    matchWord(evt);
}, false);
</script>

<tr><th>Username</th><td><input type = 'text' name='usname' id="userN' value="30" /></td><br>
<tr><th>Password</th><td><input type = 'password' name='pasew' id='Upass' value="20"/></td></tr><br>
<input type ="submit" id="submit" value="submit" />