Improve the PHP Layer in Spacemacs

Kosta H. on the emacs-php mailing list brought up [the PHP layer for Spacemacs](https://github.com/syl20bnr/spacemacs/tree/master/layers/%2Blang/php) and what we could do to improve it. Personally I have never used Spacemacs, but it's growing in popularity and I think anything we could do to improve the out-of-the-box support for PHP would be effort well-spent. So this issue is for the sake of discussing what we could do in that regard, the priorities for such efforts, who wants to volunteer to do this or that, and so on.