如何在GamemakerStudio 2中使用http_post_string将数据发送到mySQL数据库？

I want to add a row to a table in a mySQL database (hosted using godaddy), with a username & password provided from within GMS2 client.

I have been trying to use http_post_string, which runs a server-side php script.

Send json username/password, and call the PHP script from within GMS2:

var data_map = ds_map_create();
data_map[? "input_username"] = input_username;
data_map[? "input_password"] = input_password;
var str = json_encode(data_map);                        

http_post_string("https://domain/script.php",str);

script.php:

<?php
    //CONNECT
    $host = 'localhost';
    $user = 'XXX';          
    $pass = 'XXX';  
    $database = 'XXX';  
    $connection = mysqli_connect($host, $user, $pass);
    mysqli_select_db($connection,$database);

    //POST DATA
    $post_data = json_decode(file_get_contents('php://input'), true);
    $data_username = $post_data['input_username']; 
    $data_password = $post_data['input_password'];

    //DATA INSERTION
    $action = "INSERT INTO accounts (UID, username, password) 
        VALUES('', '$data_username', '$data_password')";

    mysqli_query($connection, $action);

    //DISCONNECT
    mysqli_close($connection);
?>

I've succeeded in adding a row to the database, but can't seem to send my GMS2 data as arguments... $data_username and $data_password seems to contain nothing, after accessing the $post_data array.