variable seems to be uninitialized

I am trying to get a list with a dropdown menu.

this is my code by the variable seems to be uninitialized.

echo "<td>" . '<select class="form-control" name="werkgevernaam">';
        $result = $select_db->query("SELECT werkgevernaam FROM werkgever order by werkgever_id");
        while ($row = mysqli_fetch_array($result)) {
            echo "<option ID='" . $row['werkgevernaam'] . "'>" . $row['werkgevernaam'] . "</option>";
        }
        echo '</select>';
        echo "</td >";