public function getUserlist() {
            $stmt = $this->pdo->query('SELECT last_name FROM network_user Order by last_name');
            $userlist = [];
                while ($row = $stmt->fetch(\PDO::FETCH_ASSOC)) {
                    $userlist[] = ['last_name' => $row['last_name']];
                    }
            return  $userlist;
        }


<select name="formResident">
                    <option>Select User</option>
                    <?php
                        while ($row = $userlist) {
                                echo '<option value="'.htmlspecialchars($row['last_name']).'"> </option>';
                        }
                    ?>

                </select>