city dropdown

<div class="form-group">
    <label for="">City</label>
    <select name="ct" id="ct" onChange="getPin()" class="form-control">
    <option value="0">Select City</option>
    </select>
    <span id="error11" class="alert-danger"></span>
</div>

When I am trying to fetch Pincode according to the selected city, It's
not working. Below is the file where I need to bring value. Do I need to
write value inside input text box to show the Pincode.

Pincode Inputbox

<div class="form-group">
    <label for="">Pin</label>
    <input type="text" class="form-control" name="pn" id="pin"
    autocomplete="off">
    <span id="error12" class="alert-danger"></span>
</div>

Ajax fetch Pincode

function getPin()
{
    var ct=document.getElementById('ct').value;
    var p;
    if(window.XMLHttpRequest)
    {
        p=new XMLHttpRequest();
    }
    else
    {
        p=new ActiveXObject("Microsoft.XMLHTTP");
    }
    p.open("POST","pin.php?ct_id="+ct,true);
    p.send();
    p.onreadystatechange=function()
    {
        if(p.readyState==4)
        {
            document.getElementById('pin').value=p.responseText;
        }
    }
}

pin.php page

<?php
include('connection.php');
if(isset($_POST['ct_id']))
{
$ctid=$_POST['ct_id'];
$pin_db="select * from pincode where cityId='$ctid'";
$pin_db_exe=$con->query($pin_db) or mysqli_error();
$ftch_pin=$pin_db_exe->fetch_array();
print_r($ftch_pin);
}
?>

When I am selecting the city, in Pincode input text its showing error.
Please need some help.
<p><br>Success</p><br>