Add new category and custom page into Site Administration menu - moodle

Question

I'm trying to add a new feature into site administration menu of moodle as part of a project that I am working on. Right now our customer only needs to integrate OnePay gateway into moodle. But in the future, they may require integration of other payment gateways (like PayPal) as well. So I need to create a category just for website administrators to input different types of payment. A new category should be somewhere in the "Advanced Features" menu (something like in the following screenshots)
Where to add new category
Example for onepay integration
With "Settings" page is a page that site administrator to input all necessary payment information (including customer's card number, access code, etc.). Right now I'm testing with using two of my testing pages for domestic and international payments.
Here is my code in file /lang/en/admin.php:
$string['payment'] = 'Payment';
$string['onepay'] = 'OnePay';
$string['test_onepay_domestic'] = 'Test Domestic Payment using OnePay';
$string['test_onepay_international'] = 'Test International Payment using OnePay';
In admin_custom directory, I created a path like this /payment/onepay/test/ and add 2 my testing pages into it. In folder /payment/, I added a file named menu.php. Here is my code in it.
<?php
$ADMIN->add('advancedfeatures', new admin_category('payment', get_string('payment', 'admin')));
$ADMIN->add('payment', new admin_category('payment', get_string('onepay', 'admin')));
//$domestic = new admin_settingpage('onepay', new lang_string('test_onepay_domestic', 'admin'), new moodle_url("/admin_custom/payment/onepay/test/domestic/index.php"));
//$international = new admin_settingpage('onepay', new lang_string('test_onepay_international', 'admin'), new moodle_url("/admin_custom/payment/onepay/test/international/index.php"));
$domestic = new admin_externalpage('test_onepay_domestic', get_string('test_onepay_domestic'), "$CFG->wwwroot/admin_custom/payment/onepay/test/domestic/index.php");
$international = new admin_externalpage('test_onepay_international', get_string('test_onepay_international'), "$CFG->wwwroot/admin_custom/payment/onepay/test/international/index.php");
$ADMIN->add('onepay', $domestic);
$ADMIN->add('onepay', $international);
I have attempted to include file menu.php in both files
/admin/settings/top.php
/admin/settings/server.php
But none of these works. Can anyone please tell me what should I do to get this works? Thank you very much. I really appreciate that.

Answer 1

Moodle doesn't have any sort of plugin type called 'payment', so by creating a hierarchy starting '/payment', you're already at odds with the way that Moodle works (as shown by needing to edit core files in order to add language strings and menu items).
I would recommend that you start again by creating an admin tool or a local plugin. Both of these types can have their own language strings and can add entries to the site admin menu (have a look at the settings.php inside any of the standard admin tool plugins or, for local plugins, an additional option is to create a local_MYPLUGIN_extend_settings_navigation() function). If you want to support multiple payment options, then your plugin could define a new subplugin type for each of the payment options.