I coded for expansion.

Yes I understand that. I just thought your solution might be overlooked because it looks more verbose than the other proposals, so I thought I'd show the basic form. Besides, it still works and is expandable if the same test (different variables, same values) can be used in several places.

What would you choose?

If there are many sets of data to check against, I would probably choose not to have it directly inside the code, but to import it some way or another. In which case there may be a logical reason (as in the subsets of data are logically link to each other) to gather the data in such a structure, beside being convenient.

I think not having to repeat the variable is an improvement. It makes it less cumbersome to have an explicit variable name, rather than just $a, and it means less copy/pasting which should be considered a sign that you may not be doing something the right way IMHO.