Boostrapious contact form - how to add some checkbox inputs to form

I use this contact form by Boostrapious and works well, but I can't find solution to add some checkbox inputs and send values by email. I added HTML inputs, but I need help with php code. Can anyone help me?

This is a link to this tutorial and code: https://bootstrapious.com/p/how-to-build-a-working-bootstrap-contact-form

<form id="contact-form" method="post" action="contact.php" role="form">

    <div class="messages"> </div>

    <div class="controls">

        <div class="row">
            <div class="col-md-6">
                <div class="form-group">
                    <label for="form_name">Firstname *</label>
                    <input id="form_name" type="text" name="name" class="form-control" placeholder="Please enter your firstname *" required="required" data-error="Firstname is required.">
                    <div class="help-block with-errors"></div>
                </div>
            </div>
            <div class="col-md-6">
                <div class="form-group">
                    <label for="form_lastname">Lastname *</label>
                    <input id="form_lastname" type="text" name="surname" class="form-control" placeholder="Please enter your lastname *" required="required" data-error="Lastname is required.">
                    <div class="help-block with-errors"></div>
                </div>
            </div>
        </div>
        <div class="row">
            <div class="col-md-6">
                <div class="form-group">
                    <label for="form_email">Email *</label>
                    <input id="form_email" type="email" name="email" class="form-control" placeholder="Please enter your email *" required="required" data-error="Valid email is required.">
                    <div class="help-block with-errors"></div>
                </div>
            </div>
            <div class="col-md-6">
                <div class="form-group checkbox">
                        <label for="checkboxoptions1">
                            <input type="checkbox" name="checkboxoptions[]" id="checkboxoptions1" value="checkboxoptions1">
                            Option1
                        </label>
                        <label for="checkboxoptions2">
                            <input type="checkbox" name="checkboxoptions[]" id="checkboxoptions2" value="checkboxoptions2">
                            Option2
                        </label>
                        <label for="checkboxoptions3">
                            <input type="checkbox" name="checkboxoptions[]" id="checkboxoptions3" value="checkboxoptions3">
                            Option3
                        </label>
                        <div class="help-block with-errors"></div>
                </div>          
                    
                    
                </div>
            </div>
        </div>
        <div class="row">
            <div class="col-md-12">
                <div class="form-group">
                    <label for="form_message">Message *</label>
                    <textarea id="form_message" name="message" class="form-control" placeholder="Message for me *" rows="4" required="required" data-error="Please, leave us a message."></textarea>
                    <div class="help-block with-errors"></div>
                </div>
            </div>
            <div class="col-md-12">
                <input type="submit" class="btn btn-success btn-send" value="Send message">
            </div>
        </div>
        <div class="row">
            <div class="col-md-12">
                <p class="text-muted">
                    <strong>*</strong> These fields are required. Contact form template by
                    <a href="https://bootstrapious.com/p/how-to-build-a-working-bootstrap-contact-form" target="_blank">Bootstrapious</a>.</p>
            </div>
        </div>
    </div>

</form>

and php code here

<?php
/*
 *  CONFIGURE EVERYTHING HERE
 */

// an email address that will be in the From field of the email.
$from = 'Demo contact form <demo@domain.com>';

// an email address that will receive the email with the output of the form
$sendTo = 'Demo contact form <demo@domain.com>';

// subject of the email
$subject = 'New message from contact form';

// form field names and their translations.
// array variable name => Text to appear in the email
$fields = array('checkboxoptions' => 'My options','name' => 'Name', 'surname' => 'Surname', 'phone' => 'Phone', 'email' => 'Email', 'message' => 'Message'); 

// message that will be displayed when everything is OK :)
$okMessage = 'Contact form successfully submitted. Thank you, I will get back to you soon!';

// If something goes wrong, we will display this message.
$errorMessage = 'There was an error while submitting the form. Please try again later';

/*
 *  LET'S DO THE SENDING
 */

// if you are not debugging and don't need error reporting, turn this off by error_reporting(0);
error_reporting(E_ALL & ~E_NOTICE);

try
{

    if(count($_POST) == 0) throw new \Exception('Form is empty');
            
    $emailText = "You have a new message from your contact form\n=============================\n";

    foreach ($_POST as $key => $value) {
        // If the field exists in the $fields array, include it in the email 
        if (isset($fields[$key])) {
            $emailText .= "$fields[$key]: $value\n";
        }
    }

    // All the necessary headers for the email.
    $headers = array('Content-Type: text/plain; charset="UTF-8";',
        'From: ' . $from,
        'Reply-To: ' . $_POST['email'],
        'Return-Path: ' . $from,
    );
    
    // Send email
    mail($sendTo, $subject, $emailText, implode("\n", $headers));

    $responseArray = array('type' => 'success', 'message' => $okMessage);
}
catch (\Exception $e)
{
    $responseArray = array('type' => 'danger', 'message' => $errorMessage);
}


// if requested by AJAX request return JSON response
if (!empty($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') {
    $encoded = json_encode($responseArray);

    header('Content-Type: application/json');

    echo $encoded;
}
// else just display the message
else {
    echo $responseArray['message'];