<div class="row">
  <div class="col-md-6">
    <label>Change Favorite Sport</label>
  </div>
  <div class="col-md-6">
    <button type="button" id="change_sport_btn" class="btn btn-primary btn-sm" data-toggle="collapse" data-target="#change_sport_form">Change</button>
    <div class="collapse" id="change_sport_form">
      <form method="post" id="some_id">
       <select name="select_fav_sport" class="form-control">
         <?php
            $sql = mysqli_query($conn, "SELECT sport_name FROM sport");
            while ($row = $sql->fetch_assoc()){
               echo "<option value="{$row['sport_name']}">{$row['sport_name']}</option>";
            }
         ?>
       </select>
      </form>
    </div>
  </div>
</div>

$(document).ready(function() {
$('select[name="select_fav_sport"]').change(function() {
  var select_fav_sport = $(this).val();
  $.ajax({
    url: "update_fav_sport.php",
    method: "POST",
    data: {
      select_fav_sport: select_fav_sport
    },
    success: function(data) {
      $('#some_id').html(data);
    }
  });
});

include '../db_connect.php';

if(isset($_POST["select_fav_sport"]))
{
 $query = "INSERT INTO users(fav_s) VALUES ('select_fav_sport')";
 $statement = $conn->prepare($query);
 $statement->execute(array('fav_s' => $_POST["select_fav_sport"]));

 $count = $statement->rowCount();
 if($count > 0)
 {
    echo "Favorite sport set Successfully..!";
 }
 else
 {
    echo "Data Insertion Failed";
 }
 }
 else
 {
    echo "unknown index: 'select_fav_sport'";
}

$server = "localhost";
    $user = "root";
    $password = "";
    $db = "authentication";
    $conn = new mysqli($server, $user, $password, $db);

    if ($conn-> connect_error){
        die("Connection Error!" . $conn-> connect_error);
    }