Discussion: Agree and implement CGL

After briefly talking with @mk-mxp about coding guidelines https://github.com/exercism/php/issues/652#issuecomment-1978248133 we agreed to split this out in a separate issue to follow up on this later.

We should agree on it first, though. I think tools like rector or other static code tools would be able to streamline this for us, when we have an agreement on what should be when.