I'm thinking about writing a desktop application that the GUI is made with either HTML or PHP, but the functions are run by a separate Java or python code, is there any heads up that I can look into?
You could expose data from Java or Python as JSON via GET request and use PHP to access it. There are multiple libraries for each of these languages both for writing and reading JSON. GET request can take parameters if needed.
How to use PHP/HTML as interface and Java/Python as function in background?