nzkiwi80 I've got a .net application posting a JSON variable to my PHP page. It contains a username & password in JSON format.
Weedpacket has provided some very informative links about how to handle incoming data and how to handle JSON code.
If the JSON data is supplied to your PHP script via a POST operation, then your PHP script will probably find that data in the superglobal $_POST variable.
There are cases where data is provided such that individual POST keys are not defined and you must get the 'raw POST data'. This is not well documented, but you should be able to obtain raw post data like so:
$raw_post_data = file_get_contents("php://input");
You should probably check to make sure
$raw_post_data is not empty before trying to use it.