Server-side newbie problelm

Hi. I'm just trying out DataTables and server-side processing. I created a server_side.html and processing.php files almost exactly like the examples, adding in my database server, username, password, and db info. When I try to execute it, I just get a popup stating

DataTables warning (table id = 'table_id'): DataTables warning: JSON data from server could not be parsed. This is caused by a JSON formatting error.

In Firebug, its failing in jquery-1.8.2.js, in parseJSON, at "return window.JSON.parse ( data );" The funny thing is, "data" looks to be simply my "php" file text, not any returned JSON from it.

I'm sure I'm doing something stupid, and this is a simple error, but I can't figure out what it is. Any help?

Thanks!

Don