<?php
$host = "myHost"; //hidden
$pw = "myPW"; //hidden
$username = "myUser"; //hidden
$dbname = "myDB"; //hidden

$con = mysqli_connect($host, $username, $pw, $dbname);

echo "<pre>";
print_r($_POST);
echo "</pre>";

$output = '';
if(isset($_POST["brand_id"]))
{
     if($_POST["brand_id"] != '')
     {
          $sql = "SELECT * FROM modelle WHERE brand_id = '".$_POST["brand_id"]."'";
     }
     else
     {
          $sql = "SELECT * FROM modelle";
     }
     $result = mysqli_query($connect, $sql);

     while($row = mysqli_fetch_array($result))
     {
          $output .= '<div class="col-md-3"><div style="border:1px solid #ccc; padding:20px; margin-bottom:20px;">'.$row["model_name"].'</div></div>';
     }
     echo $output;
}
?>