First off, their docs are almost entirely non-existent. File a bug, or better yet, send them a pull request.
For pings in particular:
$client->require_xep(array(
'0199' // XMPP Ping
));
If you want to write your own protocols, look at their XEP-0199 implementation for an example:
public function get_ping_pkt() {
$attrs = array(
'type'=>'get',
'from'=>$this->jaxl->full_jid->to_string(),
'to'=>$this->jaxl->full_jid->domain
);
return $this->jaxl->get_iq_pkt(
$attrs,
new JAXLXml('ping', NS_XMPP_PING)
);
}