AJAX POST Not Returning php mysql ajax

Question

I'm trying to get data from the database using ajax to insert it in other element but the post data not passing to get-data.php so what the reason can be and the solution

addBuilding.php

<?php
require_once("./dbConfig.php");
$selectIL = "SELECT * FROM  iller ";
$selectIL = $db->prepare($selectIL);
$selectIL->execute();
$res = $selectIL->get_result();
?>

<form action="" method="post">
    <select name="pp" id="cites">
        <option value="">-select state-</option>
        <?php
        while ($row = $res->fetch_assoc()) {
        ?>
            <option value="<?= $row['id'] ?>"><?= $row['il_adi'] ?></option>
        <?php
        }
        ?>
    </select>
    <select name="district" id="district">
         <option value="">-select district-</option>
    </select>
</form>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
   <script src="getdata.js"></script>

getdata.js

$(document).ready(function() { 
    $("#cites").change(function() {
        if ( $("#cites").val()!="") {
            $("#district").prop("disabled",false);
        }else  {
            $("#district").prop("disabled",true);
        }
        var city = $("#cites").val(); 
        $.ajax({ 
            type: "POST",    
            url:"get-data.php",
            data:$(city).serialize(),
            success: function(result) {
                $("#district").append(result); 
            }
        });

    });
});

get-data.php I can see the form data in network inspection put no data passing to get-data.php

<?php
require_once("./dbConfig.php"); 

if (isset($_POST['pp'])) {
   
    $cites = $_POST['cites'];
    $selectIlce = "SELECT * FROM  ilceler where il_id=? ";
    $selectIlce = $db->prepare($selectIlce);
    $selectIlce->bind_param("i", $cites);
    $selectIlce->execute();
    $res = $selectIlce->get_result();
?>
    <?php
    while ($row = $res->fetch_assoc()) {
    ?>
        <option value="<?= $row['id'] ?>"><?= $row['ilce_adi'] ?></option>
<?php
    }
}
?>

Answer 1

You need to echo the results in get-data.php

<?php
    while ($row = $res->fetch_assoc()) {
    ?>
        echo "<option value='". $row["id"]."'>".$row['ilce_adi']."</option>";
<?php
    }
}
?>

Answer 2

1- Get data by serialize from form:

$("form").serialize()

2- Add dataType: "json" to ajax option:

$.ajax({ 
    type: "POST",    
    url:"get-data.php",
    data:$(city).serialize(),
    dataType: "json",
    success: function(result) {
        $("#district").append(result); 
    }
});