mysql update from html select option error

Question

I have a simple select option to update a users information, but it doesnt seem to update it in mysql.

Have a look in my code.

session_start();
$UserID = $_GET['id'];
$UserID = preg_replace("[^0-9]", "", $UserID);

$result = mysql_query("SELECT first_name, last_name, active FROM users WHERE UserID='$UserID' LIMIT 1");
$row = mysql_fetch_assoc($result);

if (isset($_POST['submit'])) {
    $active = $_POST["active"];
    $sql= mysql_query("UPDATE users SET active = '$active' WHERE UserID='$UserID' LIMIT 1");
        $succesMsg .= "<div class='alert alert-success alert-dismissable'>
            <i class='fa fa-ban'></i>
            <button type='button' class='close' data-dismiss='alert' aria-hidden='true'>x</button>
            Account gegevens gewijzigd
            </div>";
    $_SESSION['succes'] = $succesMsg;
    header("Location: ".DEFAULT_URL_ADMIN."/user/");
} 

HTML

<form action="<?=$_SERVER['PHP_SELF'];?>" method="post" enctype="multipart/form-data" name="active_user" class="standerdForm">
                            <div class="box-body">
                                <div class="form-group">
                                        <label>Selecteer</label>
                                        <select class="form-control" id="active" name="active">
                                          <option value="0" <?php if($row['active'] == 0) {echo 'selected';} ?>>Deactiveren</option>
                                          <option value="1" <?php if($row['active'] == 1) {echo 'selected';} ?>>Activeren</option>
                                        </select>
                                </div>
                            </div>
                            <div class="box-footer">

                            <center>
                                <a href="<?=DEFAULT_URL_ADMIN?>/user/" class="btn btn-default marginRight_20">Annuleren</a>
                                <input type="submit" name="submit" id="submit" class="btn btn-primary" value="Opslaan">
                            </center>

                            </div>  
                        </form>

Answer 1

I needed to change

<?=$_SERVER['PHP_SELF']

to

$_SERVER['REQUEST_URI']