Not many devs want to work on PHP codebases, that's absolutely correct. PHP does not really look great on the resume, unfortunately.

But you should be hiring to solve a problem -- seemingly speed and reliability, in your case -- not for a language.

But yeah, tell them what stack they will be working on ;)

If you were almost anywhere else, you'd have a much easier time finding .NET developers, but then there's the whole licensing can of worms (and .NET core isn't really a viable alternative at the moment).