ajax javascript jquery php wordpress

AJAX in WordPress only returns 0

I’m trying to utilize WordPress’s admin-ajax feature in order to build a dynamic admin panel option-set for a plugin. Essentially, once an option is selected from a dropdown (select/option menu), PHP functions will sort through and display more dropdown menus that fall under the dropdown above it. I began with a simple return that I was hoping to utilize later down the line, but I can’t seem to get the text to print out without running into unidentified issues.

The AJAX I set up puts out a 200 status but the response never builds, and I’m left with 0 as my result. Here’s the code:

JS/jQuery built into PHP function ajax-action()

$ = jQuery;
$('#platform').change(function(e) {
var data = {
action: 'action_cb',
type: 'POST',
dataType: 'json',
error: function(XMLHttpRequest, textStatus, errorThrown) {
success: function(response) {
$.ajax(ajaxurl, data, function(data) {

PHP functions and add-actions

function action_cb() { $platform = 'test'; echo json_encode($platform); wp_die(); };

My question is: how can I fix this and prevent it from continuing to happen? I’d like to return the actual results and not 0.