public function __construct()
 {
     $dbinfo = MyDatabase::getConnectionDetails();
     $host = $dbinfo["host"];
     $database = $dbinfo["database"];
     $username = $dbinfo["username"];
     $password = $dbinfo["password"];
     $db = new MyDatabase($host, $database, $username, $password);
     $this->conn = $db->get_connection();
 }
            <input type="submit" name="submit" value="submit" class="btn btn-primary"/>
          </form>
        </div>
      </div>
    </div>
  </div>
  
  <?php 
if (isset($_POST["submit"])) {
    $username = trim($_POST["username"]);
    $password = trim($_POST["password"]);
    if (strlen($username) == 0 || strlen($password) == 0) {
        echo '<script type="text/javascript">' . '$(".error-message").removeClass("hidden");' . '$(".error-message p strong").html("You\'ve got an empty field");' . 'setTimeout(function() {$(".error-message").hide();}, 3000);' . '</script>';
        return;
    }
    $dbinfo = MyDatabase::getConnectionDetails();
    $host = $dbinfo["host"];
    $database = $dbinfo["database"];
    $db_username = $dbinfo["username"];
    $db_password = $dbinfo["password"];
    $db = new MyDatabase($host, $database, $db_username, $db_password);
    $conn = $db->get_connection();
    $query = "SELECT * FROM authentication WHERE username = ?";
    $stmt = $conn->prepare($query);
    $stmt->bindParam("1", $username);
    $stmt->execute();
    $result = $stmt->fetch(PDO::FETCH_ASSOC);
    /*echo(var_dump($result));*/
    if ($result == false) {
        echo '<script type="text/javascript">' . '$(".error-message").removeClass("hidden");' . '$(".error-message p strong").html("Your username or password is wrong");' . 'setTimeout(function() {$(".error-message").hide();}, 3000);' . '</script>';
    } else {