Improve syllabus

Hello maintainers 👋

I program in PHP for ~15yrs now and would like to add my share to the PHP track of Exercism. I do not have plenty of time, but could add a few concept exercises per month. Or similar.

Where should I start? Is there a plan for the syllabus? A list of concepts to have? Or something else you prepared to pick up and incorporate?

I saw #324 and maybe I should give that a try first? I'm not sure if the existing concept exercises cover enough topics to lock a good share of the practice exercises, so maybe that's better done later?