troubleshooting – WordPress all php BUT index.php downloads instead of loads

I’ve looked around a couple other solutions, but none have worked so far (modifying .htaccess, emptying .htaccess, disabling themes,plugins).

My website works fine, the index.php in the root directory works. Any other php file tries to download instead of loading. Not sure why this started happening. I haven’t logged into the admin page in a few weeks, and now its not working. The server hasnt changed at all.

I am using firefox, but have tried other browsers, the same issue occurs.
All the permissions for all php files are still under www-data and rw-r–r–

What can I do to find/fix this problem?