How to add and access custom php code in WordPress site?

I need to add some of my own php files to an existing WordPress site (from Bitnami) so it can be accessed from a 3rd-party service. I don't need to access anything from WordPress in my code, but I need my code to be accessible from a url for 3rd-party services.

Like this:

https://myWordPressSite.com. <- normal WP site

https://myWordPressSite.com/myCustomDirectory/generateSerial.php <- my code
https://myWordPressSite.com/myCustomDirectory/doSomething1.php <- my code
https://myWordPressSite.com/myCustomDirectory/doSomething2.php <- my code

How can I add my directory of code to the WordPress file structure, and how to access it from a URL?

You can add a $_GET OR $_POST to the header as a plugin for WordPress.

You will need a header of the plugin

/*
Plugin Name: name
Plugin URI: http://www.com/
Description: test
Version: 1.0
Author: your name
Author URI: http://www.com/
*/

The first part of the code would be. The code you put in here would show in the header when each page is call

function test_head() {
	echo $_GET['name it some thing']; 
	echo $_POST['name it some thing']; 			
}

To call the function you need to add this line of code

add_action( 'wp_head', 'test_head' );

To learn more about add_action you can read it at
https://developer.wordpress.org/reference/functions/add_action/

After you a Activate the plugin
You can now call your web site with GET or POST base on what you added to the header.

By doing it was a plugin you can use the WordPress backbone function

Good Luck
Karl K

Create a folder in your theme's root folder called myCustomDirectory and place your php file inside that.

then add to your functions.php the following:

This will handle the Rewrite Rules:

add_action('init', 'custom_add_rewrite_rule');
function custom_add_rewrite_rule() {
    add_rewrite_rule('^myCustomDirectory/generateSerial.php', 'index.php?generateSerial=true', 'top');
}

This will handle the WordPress Query Vars:

function add_query_vars_filter( $vars ){
    $vars[] = 'generateSerial';
    return $vars;
}
add_filter( 'query_vars', 'add_query_vars_filter' );

This will tell wordpress to pull the correct file:

function load_custom_template($template){
    if(get_query_var('generateSerial') ){
         $template = get_template_directory() ."/myCustomDirectory/generateSerial.php";
    }

    return $template;  
}

add_filter('template_include', 'load_custom_template');

make sure to flush your rewrite rules when you are done by going to: Settings > Permalinks and clicking 'save changes'

It turns out it was much simpler than I thought, or any of the other answers presented here.

The answer is to just add a directory of my own code inside the WordPress home folder.

In the case of a AWS/Lightsail/Bitnami/Wordpress package, the home directory is:

/opt/bitnami/wordpress

So I created a directory and put my code in it..

/opt/bitnami/wordpress/myDirectory
/opt/bitnami/wordpress/myDirectory/generateSerial.php
/opt/bitnami/wordpress/myDirectory/doSomething.php

And it was then accessible from a url like:

https://myWordPressSite
https://myWordPressSite/generateSerial.php //etc

One small catch is that some WordPress packages and some other Bitnami packages are laid out a bit different, so check on your specific installation.