<?php

require_once 'config.php';

require_once $CFG->dirroot . '/lib/formslib.php';

class myform extends moodleform {

    public function definition()

    {

        $this->_form->addGroup([

            $this->_form->createElement('advcheckbox', 'c1', 'off'),

            $this->_form->createElement('text', 't1'),

            $this->_form->createElement('date_selector', 'd1'),

        ], 'group');

        $this->_form->addElement('date_selector', 'd2');

        $this->_form->disabledIf('group[t1]', 'group[c1]', 'checked');

        $this->_form->disabledIf('group[d1]', 'group[c1]', 'checked');

        $this->_form->disabledIf('d2', 'group[c1]', 'checked');

        $this->add_action_buttons();

    }

}

$PAGE->set_url(new moodle_url($_SERVER['REQUEST_URI']));

$PAGE->set_context(context_system::instance());

$page = optional_param('page', 'index', PARAM_TEXT);

echo $OUTPUT->header();

$form = new myform();

$form->display();

echo $OUTPUT->footer();