Checking post data of a checkbox - php

Question

I have a checkbox that I'm looking to update a contacts details with. At the moment it's not updating the contact in Send in Blue... I don't think that's the problem though.
There's 3 issues I'm not sure I'm doing correctly:
Checking if the data has been sent.
Checking the field data that already uses square brackets.
Checking to see if the checkbox is checked or not.
Here's my php:
<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
global $SendInBlue;
if(isset($_POST['data[newsletter_sub]']) == "on") {
$data_in['ONETIME_NEWSLETTER'] = true;
$data_in['PROMOS'] = true;
$data_in['SUB_NEWSLETTER'] = true;
} else {
$data_in['ONETIME_NEWSLETTER'] = false;
$data_in['PROMOS'] = false;
$data_in['SUB_NEWSLETTER'] = false;
}
// Update SiB contact
$data = array(
'attributes'=> $data_in
);
try {
$updateContact = new \SendinBlue\Client\Model\UpdateContact($data);
$result = $SendInBlue->updateContact($email, $updateContact);
return true;
} catch (Exception $e) {
}
}
?>
Here's some of the form with one checkbox:
<form method="POST" class="" id="communication_preferences" action="/event" name="communication_preferences">
<input type="checkbox" name="data[newsletter_sub]" id="newsletter_sub" <?php if($newsletter_sub == 'yes'){ echo 'checked';}?>>
<button class="button button--pink button--uppercase text--three js-profile-item-submit landmark ">
Save Changes
</button>
</form>
The page just reloads when you submit the form.

Answer 1

The problem with checkboxes is that they do not appear in the $_POST when not checked.
Your receiving script can simple do this:
$bMyCheckOn = isset($_POST["someCheckBoxName"]); // true/false

Answer 2

Please make sure that if (!empty($_POST['newsletter_sub'])) { matches your input.
Here, i rewrote your entire script:
<?php
if (!empty($_POST['submit'])) {
global $SendInBlue;
$data_in = [];
foreach(['ONETIME_NEWSLETTER', 'PROMOS', 'SUB_NEWSLETTER'] as $data) {
if (!empty($_POST['newsletter_sub'])) { //Checkbox set?
$data_in[$data] = true;
} else {
$data_in[$data] = false;
}
}
$data = ['attributes' => $data_in];
$attempt = new \SendinBlue\Client\Model\UpdateContact($data);
$result = $SendInBlue->updateContact($email, $attempt) ? true : false;
print_r($result);
} else {
echo 'Form not submitted.';
}
?>
<form method="POST" id="communication_preferences" action="">
<input type="checkbox" name="newsletter_sub" id="newsletter_sub" <?=($newsletter_sub == 'yes' ? 'checked' : '')?>>
<button type="submit" class="button button--pink button--uppercase text--three js-profile-item-submit landmark" name="submit" value="submit">
Save changes
</button>
</form>