public function definition() {
        $mform = $this->_form;
        $mform->addElement('hidden', 'returnurl', $this->_customdata['returnurl']);
        $mform->setType('returnurl', PARAM_LOCALURL);
        $mform->addElement('submit', 'all', get_string('purgecaches', 'admin'));
        $mform->addElement('header', 'purgecacheheader', get_string('purgeselectedcaches', 'admin'));
        $checkboxes = [
            $mform->createElement('advcheckbox', 'theme', '', get_string('purgethemecache', 'admin')),
            $mform->createElement('advcheckbox', 'lang', '', get_string('purgelangcache', 'admin')),
            $mform->createElement('advcheckbox', 'js', '', get_string('purgejscache', 'admin')),
            $mform->createElement('advcheckbox', 'filter', '', get_string('purgefiltercache', 'admin')),
            $mform->createElement('advcheckbox', 'muc', '', get_string('purgemuc', 'admin')),
            $mform->createElement('advcheckbox', 'other', '', get_string('purgeothercaches', 'admin'))
        ];
        $mform->addGroup($checkboxes, 'purgeselectedoptions');
        $mform->addElement('submit', 'purgeselectedcaches', get_string('purgeselectedcaches', 'admin'));
    }