$.post('/', {a:100, b:'test'}, function(data){
}, 'json');

Content-Type    application/x-www-form-urlencoded; charset=UTF-8

PHP 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

json_decode(file_get_contents("php://input"));

PHP Warning:  First parameter must either be an object or the name of an existing class

a=100&b="test"

parse_str(file_get_contents("php://input"), $data);
$data = json_decode(json_encode($data));
var_dump($data);