> Being able to do SSR and sharing validation logic are two huge and obvious reasons. Ever try to exactly duplicate validation rules between the browser and PHP? What about when they use weird regexp rules?

PHP is adding support for FFI to any C ABI-compatible language, and you can run the exact same code in the browser via emscripten or a WASM polyfill. As a bonus, emscripten/WASM compiled code will also be more efficient in the browser than plain JavaScript or Typescript.

> As a bonus, emscripten/WASM compiled code will also be more efficient in the browser than plain JavaScript or Typescript.

Small gains compared to the cost of implementing. 10-20%. Now you're asking your full-stack developers to know PHP, JS, C, and WASM.

It seems silly to add a third language to your stack just so you can share code between the other two languages.

Here is my perspective: There are plenty of reasons to use the same language on FE and BE but for me the main problem is the JS standard library.

Let me give you one example:

Lets say you want to validate if a string is a valid URL. What is a good way to do this in JS?

Just for reference in Ruby for example I can do:

   URI.parse(input).kind_of?(URI::HTTP)

No gems or any other engines needed.

The URL constructor.

Oh wow, how I missed that. I stand corrected!

Seems like JS standard library is better than I thought.