Why am I getting a 403 Forbidden error?

This usually means your nonce verification failed. Ensure that the action string passed to wp_create_nonce('my_action') perfectly matches the string passed to check_ajax_referer('my_action', 'security'). Also, verify that the logged-in user generating the nonce is the same user executing the AJAX call, as nonces are tied to user sessions.

Why does my AJAX call return a "0" instead of JSON?

If your response body is simply `0`, it means your PHP function executed, but it reached the end of the `admin-ajax.php` file without firing wp_die(), wp_send_json_success(), or wp_send_json_error(). Always ensure your custom handler terminates the script explicitly.

Why is my POST data missing or empty on the server?

When using the Fetch API instead of jQuery, sending data as `application/json` won't automatically populate the `$_POST` array in PHP. You must use the `FormData` object in JavaScript (as demonstrated in Step 3) or manually read the `php://input` stream if you prefer sending raw JSON strings.

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