Is it possible to create PHP extensions in Haskell?
You can certainly do this, though I'm not sure anyone has tried. Haskell interoperates with C via its FFI mechanism, and you can certainly:
so if you can call C from PHP, and that C calls Haskell, you're in business.
Why would you want to do this? Faster, safer, simple code for your core algorithms or safety-critical code, perhaps. Or to take advantage of say, a parallel or multicore server backend via Haskell.
Yes, its possible. Just make sure you wrap everything and translate arguments between php -> c -> haskell and the way back correctly.