I want to run a lightweight server hosting a website and running some basic PHP and C++ for a project.
I have little to no experience with servers (I have used Xampp to run a local server a few times) however am happy to learn (just don't make things too difficult).
I am looking for OS and software recommendations.
Some guides for setting up the server would be nice as well but I'm sure I can find them myself.

Hardware
I plan to run this on an old PC I have lying around, with ~3GB RAM and a Core 2 Duo.

Preferences
I would like this server to be power efficent and use very little resources (e.g. If possible I'd prefer to use a CLI as it will be faster and less resource intensive than a GUI) however still relatively reliable (Doesn't have to be fast to load though).
I would like this server to completely run on open source software (if possible).

Thank you

Slackware or Debian are possibilities. Or one of the BSDs. A few months ago I'd have added Centos, which was the community version of Red Hat, but I believe that's an orphan project now.

Any Linux can be run without a GUI and on servers it usually is.