Toggle checkboxes on PHP table

I have a table with a checkbox in the table header, which will be used to toggle all the checkboxes below it. I've add a JavaScript function to the checkbox in the header, but so far it only selects the top checkbox (one checkbox) instead of all of them. Any suggestions or advice on what I've done wrong or what to research?

HTML table header code:

<thead>
  <tr>
    <th><center><input type="checkbox" value="" id="cbgroup1_master" onchange="togglecheckboxes(this,'cbgroup1')"></center></th>
    <th>Sender</th>
    <th>Receiver</th>
    <th>Subject</th>
    <th>Date</th>
  </tr>
</thead>

PHP table code:

$table .= '
          <tr>
            <td><center><input type="checkbox" id="cb1_1" class="cbgroup1"></center></td>
            <td><a href="pm_message.php?u='.$log_username.'&pmid='.$pmid.'" onclick="markRead(\''.$pmid.'\',\''.$sender.'\')">'.$sender.'</a></td>
            <td>'.$receiver.'</td>
            <td>'.$subject.'</td>
            <td>'.$time.'</td>
          </tr>';