I meant it in good humor, but over the last 15 years, I have had the joy (and pain) of getting to ship production systems in a lot of less popular stacks including: Erlang (multiple), Eiffel, Haskell, and OCaml.
Hiring and training for these less popular stacks has been a goddamn nightmare in "win" conditions. You have the benefit of the better signal to noise ratio in resumes, absolutely. But, you pay for that in rarity, salary and job demands, which is fine as long as you go in eyes wide open and plan to have a small team (under 10).
Rapid team growth (from money injection, or simply success and need) is really, really hard when you don't have a large pool of qualified candidates to pull from... and training, I honestly can't fathom trying to train PHP developers in Haskell. If you company has done this successfully, please setup a newsletter, I would subscribe.