Php - How To Embed The Result Of A Webpage Into Another Webpage On Another Website

I would like to allow users to call my ruby on rails app as a service which returns a 'div' with html content in it, and embed that div into their app (which will not be a rails application).

For example, assume someone has their own php website that has a header/footer template that gets rendered, and a content area of the page that they need to fill based on some html I generate in my rails app. I would like to allow them, from php, to call to my website, get the 'div' I generate, and embed that as html in their php page.

What I'm trying to do is host a service on my site that returns some html content, but actually show that content as part of another site, so that the end user only sees the other site and never really knows about mine.

Also, I can use javascript on the client to do this if that is the only way, but I would prefer the php app to handle this at the server if possible so the client gets the html embedded from the original server and it looks like it all was generated by the php script that generated the entire page. I also want to avoid using an iframe.