404 returned on all pages within App other than the Index

[Milkman1]

I feel like I'm missing an elementary step. I am building out an app in PHP. 

directly calling mydomain.com/myapp/products.php inside a browser, shows the page correctly. However, within the iframe of my Shopify store, whenever I try to navigate to another page other than index.php, I get a 404 error.

I can however, require_once or include other scripts in the directory. Just not pull them up directly.

Do I need to somehow whitelist all other pages the Shopify window is allowed to navigate to?