如何在主题中从外部URL加载functions.php

We have this serious issue in WordPress. We have this problem that clients download all the files from the server and uploads it to his. Now we want to do this.

1. Host the functions.php on our server. The entire code. Lets call this file (FILE A) an it is on ABC.com
2. In the themes folder's function.php file ( FILE B) , we want the FILE A to be rendered .

We tried to directly include FILE A into FILE B, but it is not possible as many servers dont allow this to happen.

Now we want a mechanism where in if someone downloads the files from our server without out permission, then we just comment the code in FILE A and the website becomes disfunctional.

Can anyone give me a way to do this?

A way to load the external file's code into theme's functions.php

I will really be obliged

Thanks in advance