Is it possible to use an external PHP script in an app?

[vincir]

Hello,

I'd like to add informations on the Thank you page that provide from my external server. For example a link to an invoice that I generate on my webserver.

Is it possible to do that : a call to an external php script on my webserver, that will return some text information that can be show on the Thank you page ?

I think that if it's possible, it's by creating an app, but I can find information about it in the documentation.

Thanks for your help
Sincerely,
Vincent