a checkbox group is a collection of checkboxes, all with the same name. For a checkbox group to work correctly, the name needs to end with []. for example:
<label>
<input type="checkbox" name="checkGroupA[]" value="1" id="checkGroupA_0" />
One</label>
<br />
<label>
<input type="checkbox" name="checkGroupA[]" value="2" id="checkGroupA_1" />
Two</label>
<br />
<label>
<input type="checkbox" name="checkGroupA[]" value="3" id="checkGroupA_2" />
Three</label>