自动填充表单不起作用

I want to make form, if i fill the first input (i.e 'rollnumber') , i want the rest of the input will filled automatically with data from mysql database (if the 'rollnumber' i filled is found in the database) And if the 'rollnumber' not found in database it will say "Rollnumber not found". How to achieve that goal?

The results with below code are:

The autofill not working, data won't show even i fill the right 'rollnumber'
the only thing that works is the #loading1, it show after i fill the data, but it won't hide back.

In case someone is kind enough to help me try my code to see what is wrong, here is the database (database name: login):

These are my codes so far:

Form HTML:

<div class="form-group">
    <input type="text" name="rollnumber" id="rollnumber" tabindex="1" class="form-control" placeholder="Roll Number" value="">
    <img src="ajax-loader.gif" id="loading1"></img>
</div>
<div class="form-group">
    <input type="text" name="fname" id="fname" tabindex="1" class="form-control" placeholder="First name1" value="">
</div>
<div class="form-group">
    <input type="text" name="lname" id="lname" tabindex="1" class="form-control" placeholder="Last name" value="">
</div>
<div class="form-group">
    <input type="email" name="email" id="email" tabindex="1" class="form-control" placeholder="Email Address" value="">
</div>
<div class="form-group">
    <input type="text" name="phone" id="phone" tabindex="1" class="form-control" placeholder="Phone">
</div>
<div class="form-group">
    <input type="text" name="batch" id="batch" tabindex="1" class="form-control" placeholder="Batch">
</div>
<div class="form-group">
    <input type="text" name="lclass" id="lclass" tabindex="1" class="form-control" placeholder="Class">
</div>

Javascript:

$(document).ready(function()
{
    $("#loading1").hide();
    $("#rollnumber").change(function()
    {
            $("#loading1").show();
            var id = $("#rollnumber").val();
            var data = 'one=' + id;
            $.ajax
            ({
                type: "POST",
                url: "checkrollnumber.php",
                data: data,
                dataType: 'json',
                success: function (data) 
                {
                    $("#loading1").hide();
                    if (data) 
                    {
                        for (var i = 0; i < data.length; i++) { //for each user in the json response
                            $("#fname").val(data[i].fname);
                            $("#lname").val(data[i].lname);
                            $("#email").val(data[i].email);
                            $("#phone").val(data[i].phone);
                            $("#batch").val(data[i].batch);
                            $("#lclass").val(data[i].lclass);
                        } // for

                    } // if
                } // success
            }); // ajax
    });
});

checkrollnumber.php:

require_once "conn.php";
header('Content-type: application/json; charset=utf-8');
if(isset($_POST['one'])){
$json = array();
$id =  trim($_POST['one']);
$query = "SELECT fname, lname, email, phone, batch, lclass FROM users WHERE rollnum = ?";
$stmt = $DB_con->prepare($query);
    $stmt->bind_param('s', $id);
    $stmt->execute();
    $stmt->bind_result($nFname, $nLname, $nEmail, $nPhone, $nBatch, $nLclass);
while ($stmt->fetch()){
   $roll=array('fname'=>$nFname,'lname'=>$nLname,'email'=>$nEmail,'phone'=>$nPhone,'batch'=>$nBatch,'lclass'=>$nLclass);
    array_push($json,$roll);
}
echo json_encode($json, true);

 }

conn.php (connection)

$DB_host = "localhost";
$DB_user = "root";
$DB_pass = "";
$DB_name = "login";


try
{
    $DB_con = new PDO("mysql:host={$DB_host};dbname={$DB_name}",$DB_user,$DB_pass);
    $DB_con->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
}
catch(PDOException $e)
{
    $e->getMessage();
}

You should return false if the rollnumber isn't in the database, so to do that you could check if the array is empty or not using count(), replace the following line :

echo json_encode($json, true);

By :

if( count($json) == 0){
     echo json_encode("false", true);
}else{
    echo json_encode($json, true);
}

Then is your JS code you should add a condition to show "Rollnumber not found" like :

$(document).ready(function(){
  $("#loading1").hide();

  $("#rollnumber").on('input', function(){
    $("#loading1").show();
    var id = $(this).val();

    $.ajax({
      type: "POST",
      url: "checkrollnumber.php",
      data: {one: id},
      dataType: 'json',
      success: function (data) 
      {
        if (data == 'false') 
        {
            alert("Rollnumber not found");
        }else{
          for (var i = 0; i < data.length; i++) { //for each user in the json response
            $("#fname").val(data[i].fname);
            $("#lname").val(data[i].lname);
            $("#email").val(data[i].email);
            $("#phone").val(data[i].phone);
            $("#batch").val(data[i].batch);
            $("#lclass").val(data[i].lclass);
          } // for

        } // if

        $("#loading1").hide();
      } // success
    }); // ajax
  });
});

NOTE : The data parameter should be sent like data: {one: id}. I suggest also the use of input as event since it's more efficient when you track the use inputs :

$("#rollnumber").on('input', function(){

Hope it will help you.

Don't declare variable similar to keyword, as in your case you declared variable data, which is confusing with data keyword in ajax.

var data = 'one=' + id;

Also, change below line of code

 data: data,

 data: {one : $("#rollnumber").val() },