Submenu Page under Settings in Admin Panel

Question

I have a quick question, I am wanting to add in a new submenu to the 'Tools' section on my Admin Panel but without modifying any of the original WordPress files such as (functions.php, etc..) files. So I'm unable to use the functions.php file to input the function call directly.

File Path:
wp-content/mu-plugins/fh-extensions/_functions/actions/instagram.php

Here is the code that I am using:

<?php
/**
 * Created by PhpStorm.
 * User: smajlovs
 * Date: 2019-08-14
 * Time: 10:44
 */

/*
* Add in a new submenu to the 'Tools.php' settings
*/
function admin_menu() {
    add_action('admin_menu', 'register_my_custom_submenu_page');

    function register_my_custom_submenu_page() {
        add_submenu_page(
            'tools.php',
            'Submenu Page',
            'My Custom Submenu Page',
            'manage_options',
            'my-custom-submenu-page',
            'my_custom_submenu_page_content' );
    }

    function my_custom_submenu_page_content() {
        ?>
            <div class="wrap">
            <h2>Page Title</h2>
            </div>
        <?php
    }
}

Could anyone let me know why it's not showing up under 'Tools' in the admin panel?

Answer 1

I didn't test this but i'm pretty sure you just need to remove your code from the admin_menu() function declaration.

Like this:

add_action('admin_menu', 'register_my_custom_submenu_page');

function register_my_custom_submenu_page() {
    add_submenu_page(
        'tools.php',
        'Submenu Page',
        'My Custom Submenu Page',
        'manage_options',
        'my-custom-submenu-page',
        'my_custom_submenu_page_content' );
}

function my_custom_submenu_page_content() {
    ?>
        <div class="wrap">
        <h2>Page Title</h2>
        </div>
    <?php
}