advcheckbox in moodle form is storing only 0 in database

Question

I am new Moodle and working on form data saving. I have created a group of checkbox which will take some input from the users and store it to the database, but unfortunately it is storing only 0/1 in the database. I have no clue where I made the mistake. This my php file where I added the form field-

class custom_signup_form extends moodleform {

    //Add elements to form
    public function definition() {
        global $CFG;
        global $DB;
        $mform = $this->_form; // Don't forget the underscore! 

        //$mform->addElement('text', 'email', get_string('email')); // Add elements to your form.
        //$mform->setType('email', PARAM_NOTAGS);                   // Set type of element.
        //$mform->setDefault('email', 'Please enter email');        // Default value.

        if($this->content !== Null){
            return $this->content;
        }

        
        $courses = $DB->get_records('course');
        $categories = $DB->get_records('course_categories');


        $interestedcourse=array();
        foreach($courses as $course){
            $coursestring = $course->fullname;
            $interestedcourse[] =  $mform->createElement('advcheckbox', 'interestedcourse[]','', $coursestring, array('group' => 1), array('',$coursestring));
        }
        $mform->addGroup($interestedcourse, 'interestedcoursegroup', get_string('interestedcourse', 'assignsubmission_metadata'),array('<br>'), false);

        $interestedcategory=array();
        foreach($categories as $category){
            $categorystring = $category->name;
            $interestedcategory[] =  $mform->createElement('advcheckbox', 'interestedcategory[]','', $categorystring, array('group' => 1), array('',$categorystring));
        }
        $mform->addGroup($interestedcategory, 'interestedcategorygroup', get_string('interestedcategory', 'assignsubmission_metadata'),array('<br>'), false);

        $this->add_action_buttons();

This is the php file for saving form data-

require_once(__DIR__ . '/../../config.php');
require_once($CFG->dirroot . '/local/custom_signup/classes/form/edit.php');

global $DB;


$PAGE->set_url(new moodle_url('/local/custom_signup/signupform.php'));
$PAGE->set_title(get_string('custom_signup', 'local_custom_signup'));

#custom form for signup

$mform = new custom_signup_form();

if ($mform->is_cancelled()) {
    // Go back to manage.php page
    redirect($CFG->wwwroot . '/local/custom_signup/manage.php', get_string('cancelled_form', 'local_message'));

}
else if ($fromform = $mform->get_data()) {

    var_dump($fromform);
    die;

}


echo $OUTPUT->header();

$mform->display();
echo $OUTPUT->render_from_template('local_custom_signup/signupform', $templatecontext);

echo $OUTPUT->footer();

object returned from the form submission-

object(stdClass)#288 (3) { ["interestedcourse"]=> array(1) { [""]=> string(0) "" } ["interestedcategory"]=> array(1) { [""]=> string(0) "" } ["submitbutton"]=> string(12) "Save changes" }