$HTTP_RAW_POST_DATA is deprecated

Deprecated: Automatically populating $HTTP_RAW_POST_DATA is deprecated and will be removed in a future version. To avoid this warning set 'always_populate_raw_post_data' to '-1' in php.ini and use the php://input stream instead. in Unknown on line 0

I got this error today, so the way i was able to fix it is with the following code:
if(!$_GET = file_get_contents("php://input"))die(';('); // get the $_POST data, this will be in json format $_GET = json_decode($_GET, true); // make post data from json to array
as you can see, you will need to use json_decode to get the data

The Solution:

all you need to do is make sure you are sending 'a' with GET method like this: $_GET['a']