How can I check if all checkboxes have been checked in PHP?

I am implementing a list of items and each has a checkbox. I currently can see which checkboxes have been checked but what I want to do is check if all of them have been checked. How can I implement that?

Here is my code:

<form action="" method="post">
     <?php
                echo "<table>
                    <tr>
                    <th>Customer ID</th>
                    <th>Report ID</th>
                    <th>Report message</th>
                    <th>Device</th>
                    <th>Device no.</th>
                    <th>Barcode</th>
                    <th>IMEI</th>
                    <th>Sale-date</th>
                    </tr>";

                while ($row2 = $clientUsername->fetch_assoc()) {

                    $_SESSION['cl_username'] = $row2["username"];
                    while ($row = $message->fetch_assoc()) {

                        $_SESSION['accept'] = $row["acceptance"];
                        $_SESSION['client_comment'] = $row["message"];
                        $_SESSION['name'] = $row["name"];
                        $_SESSION['sales_date'] = $row["sales_date"];
                        $_SESSION['date_sent'] = $row["date_sent"];


                        $_SESSION['countable_array']  = $row;

                        ?>

                <?php if ($row['acceptance'] == 3) {

                            echo "<tr> <td>
                                  " . '<input type=checkbox name=devices[] value=' . $row['dev_id'] . '>' . "</td> <td>" . $cus_id . "</td> <td>" . $rep_id . "</td> <td>" . $_SESSION['client_comment'] . "</td> <td>" . $_SESSION['name'] . "</td> <td>" . $row["device_no"] . "</td> <td>" . $row["barcode"] . "</td> <td>" . $row["serial_imei"] . "</td> <td>" . $row["serial_no"] . "</td> <td>" . $row["sales_date"] . "</td></tr>";
                            echo "</table>";
                        }
                    }
                }
</form>

    if ($_SERVER['REQUEST_METHOD'] == "POST"&& isset($_POST['rejected'])) {
 if (count($count_devices) == 1) {
...
}
}