require at least one checkbox?

I have a form with a group of checkboxes where the user must select at least one, but can select more than one in any combination. I have no idea how to set up validation to require at least one of them. Below is the form code I'm using. Thanks!

<div class="lineGroup">
    <label class="checklabel" for="study_print"><input type="checkbox" name="study_print" id="study_print" value="yes" class="formCheckboxField_Standard" <?php if (!(strcmp((isset($_GET["invalid"])?ValidatedField("rrf2017insert2","study_print"):""),"yes"))) {echo "checked=\"checked\"";} ?> tabindex="27" title="Please enter a value."> Print</label>
    <label class="checklabel" for="study_digital"><input type="checkbox" name="study_digital" id="study_digital" value="yes" class="formCheckboxField_Standard" <?php if (!(strcmp((isset($_GET["invalid"])?ValidatedField("rrf2017insert2","study_digital"):""),"yes"))) {echo "checked=\"checked\"";} ?> tabindex="27" title="Please enter a value.">Digital</label>
    <label class="checklabel" for="study_combined"><input type="checkbox" name="study_combined" id="study_combined" value="yes" class="formCheckboxField_Standard" <?php if (!(strcmp((isset($_GET["invalid"])?ValidatedField("rrf2017insert2","study_combined"):""),"yes"))) {echo "checked=\"checked\"";} ?> tabindex="27" title="Please enter a value.">Print + Digital</label>
</div>

You would need some hand coding to get it working. Here is some sample code that should work. Just add this before the server validation code:

<?php

$checkedBoxes = 0;

if (isset($_POST["study_print"])) $checkedBoxes++;

if (isset($_POST["study_digital"])) $checkedBoxes++;

if (isset($_POST["study_combined"])) $checkedBoxes++;

?>

Then you can validate the value of <?php echo($checkedBoxes); ?>

Using number validation to validate that it is greater than zero.

Thanks a lot, Ray. I added the code above where it appears the server validation begins on my page (see below). Though, I'm not quite sure about the second part you mention "Using number validation..." Could you explain that to this php noob?

Open your server validation and for the validated field put: <?php echo($checkedBoxes); ?>

and for the validation type choose number validation.

Then click the add button and set the number of checkboxes required.

Hey, can we set up a zoom call for you to help me with this? I'm getting all cross-eyed looking at it.

Just call the number at the bottom of the website and I can help.

I just followed this thread to do the same thing, however one correction needed to get it working - instead of putting <?php echo($checkedBoxes); ?> in the Server Variable box, just put $checkedBoxes otherwise it'll throw up a PHP error. Thanks for the instructions though, did exactly what I needed!