As specified from the RFC4627 the correct header is application/json and not text/json.
In PHP you can use:
header('Content-type: application/json');
If you need not cache the output, a common problem with GET requests on IE, you can use these lines of code:
header('Cache-Control: no-cache, must-revalidate');
header('Expires: Mon, 26 Jul 1997 05:00:00 GMT');
header('Content-type: application/json');
