The high level declarative nature and type driven development style of languages like Haskell also make it really easy for an experienced developer to review and validate the output of the LLM.
Early on in the GPT era I had really bad experiences generating Haskell code with LLMs but I think that the combination of improved models, increased context size, and agentic tooling has allowed LLMs to really take advantage of functional programming.
Well, you can adapt your PHP producing pipeline to produce Haskell code that is correct in the sense of solving the problem at hand, but getting it to produce idiomatic code is probably a lot harder.