if($_SERVER["REQUEST_METHOD"] == "POST") {
  // username and password sent from form

  $username = mysqli_real_escape_string($link,$_POST['username']);
  $password = mysqli_real_escape_string($link,$_POST['password']);

  $sql = "SELECT ID_Klient FROM klienti WHERE Username = '$username' and
   Password = '$password'";
  $result = mysqli_query($link,$sql);

  $row = mysqli_fetch_array($result,MYSQLI_ASSOC);
  $active = $row['active'];

  $count = mysqli_num_rows($result);
  if($count == 1) {
    $_SESSION['username'] = $username;
    $_SESSION['success'] = "Jeni i loguar";
     header('location: clientmainpage.php');
     }else {
     $error = "Your Login Name or Password is invalid";
   }
  }
 ?>

session_start();

$data = date('d/m/Y');
 $bileta = mysqli_real_escape_string($link, $_REQUEST['numri_bileta']);
 $pagesa = mysqli_real_escape_string($link, $_REQUEST['pagesa']);
 $id_udhetimi=mysqli_real_escape_string($link, $_REQUEST['id_udhetim']);
 $username= $_SESSION['username'];
 $query="SELECT ID_Klient FROM klienti WHERE Username=$username";
 $id_klienti= mysqli_query($link,$query);

 $sql = "INSERT INTO rezervimi (Date_Rez, N_Bileta, Paguar,
 ID_Klient,ID_Udhetim) VALUES ('$data', '$bileta',
 '$pagesa','$id_klienti','$id_udhetimi')";
 if(mysqli_query($link, $sql)){
   header('location: clientmainpage.php');
 } else{
  echo "ERROR: Could not able to execute $sql. " . mysqli_error($link);
 }