Controlling PHP Output

I'm working on a web page that has a div that contains a slide show.  The source of the page is a database record on the server that contains the  type of slide show, pointers to the images that will be displayed, and perhaps a half dozen other slide show parameters that the user will set through their browser by interacting with a JavaScript dialog. The idea is that the slide show parameters will be Ajaxed up to the server, the server will build the new HTML for the player div, and the new innerHTL for the player div will be Ajaxed back down.

What I'm not seeing clearly is how I can get PHP to build up the player div HTML without outputting anything automatoca;;u to the client.  That is, normally when a browser requests a PHP page, the PHP runs and whatever is left automatically shows up in the client' browser.  I don't want that.  I just want what the innHTML for one div to show up and this only because we return it in a Ajax request.

Thanks for any thought.
Steve