Try this:
$postData = json_decode(file_get_contents("php://input"));
If you simply POST a good old HTML form, the request looks something like this:
POST /page.php HTTP/1.1
key1=value1&key2=value2&key3=value3
But if you are working with Ajax a lot, this probaby also includes exchanging more complex data with types (string, int, bool) and structures (arrays, objects), so in most cases JSON is the best choice. But a request with a JSON-payload would look something like this:
POST /page.php HTTP/1.1
{"key1":"value1","key2":"value2","key3":"value3"}
The content would now be application/json (or at least none of the above mentioned), so PHP's $_POST-wrapper doesn't know how to handle that (yet).
The data is still there, you just can't access it through the wrapper. So you need to fetch it yourself in raw format with file_get_contents('php://input')
(as long as it's not multipart/form-data-encoded
).