I have a GWT app and I'm using RequestBuilder get GET some json from a php script I have running on a Fatcow.com server. It returns the json just fine in the browser and returns a 200 status in Charles web debug proxy, but in the GWT app it always says the response status is 0 and doesn't give me any json. When I test my code on a known working URL, it returns 200 and I get the json I expect. Also, I already have header('Content-Type: application/json; charset=utf-8'); in my php, which I know is a common error. Is there any reason this would not be working? Is it a php thing, or am I doing something wrong?
You're hitting the Same-Origin Policy.
CORS is supported in most recent browsers (exceptions: IE and Opera; will be coming in IE 10 and Opera 12 respectively).
Only viable alternatives are JSONP (using JsonpRequestBuilder in GWT) or a "proxy" on the same server serving your GWT app.