public function upgradekey_form_page($url) {

        $output = '';
        $output .= $this->header();
        $output .= $this->container_start('upgradekeyreq');
        $output .= $this->heading(get_string('upgradekeyreq', 'core_admin'));
        $output .= html_writer::start_tag('form', array('method' => 'POST', 'action' => $url));
        $output .= html_writer::empty_tag('input', array('name' => 'upgradekey', 'type' => 'password'));
        $output .= html_writer::empty_tag('input', array('value' => get_string('submit'), 'type' => 'submit'));
        $output .= html_writer::end_tag('form');
        $output .= $this->container_end();
        $output .= $this->footer();

        return $output;
    }