How to add file to localhost from another computer

I have a XAMPP-hosted MySQL database on one computer, and I can access it on this computer using the other computer’s IP address. There’s a couple of php files that I can use to interact with the database, and they work fine on this computer. However, in the same directory as those php files (xampp/htdocs/demo/) I want to add another php file, but I want to build it on this computer. There’s tons of stuff out there on accessing localhost on another computer (which I can do no problem) but not much on modifying the localhost directory from another computer.

So, is either of these two things possible:

1. Actually adding a php file to the localhost directory from this computer (ideal) OR

2. Creating a php file on this computer (not on localhost) that can still access and modify my database.

?

Thanks! (and let me know if you need more information.)