Hi,
xRTML uses the Realtime Messaging service and was mainly built for JavaScript usage thought it's JavaScript library.
Since it has a special message format and no PHP library you need to send a "raw" Realtime message using the xRTML message format through the Realtime Messaging PHP.
(I'm assuming you have no issues with sending a Realtime message using the PHP SDK)
For the example in your question you need to send the following JSON message to the channel myChannel:
{"xrtml":{"t":"myTrigger","a":"","d":{"title":"Title - HTML Toast","text":"Text - HTML Toast","displayBanner":true,"bannerType":"html","destinationUrl":"
http://www.xrtml.org","bannerContent":"<div><center>This is a test.</center></div>","metaData":"RandomMetaData"}}}
Let me know if this worked for you.
Regards
Joao