How do you call app functions

[SidCQ]

For example, this is the basic shopify app supplied as an example:

https://github.com/uniacid/PHP-Shopify-Plugin/blob/master/Shopify.php

Say I have a function: public function get_products() in my app.

How do I call this from a shopify page when it loads?
For example, when you open an order from your account page, I want to call a php function in my app but I can find no documentation on how to do this. Any help would be appreciated.