Can we talk about "reproducible builds" when working with interpreted languages?
A "build" does not have though to be a compilation step.

".. the term build is similar to that of any other field. That is, the construction of something that has an observable and tangible result."
https://en.wikipedia.org/wiki/Software_build

More important in this context is to be able to reuse results of your build in other contexts to build new things.

That is, also in PHP and Javascript you could produce some packaged artefacts, be it a zip file or language specific archive linkable to a dependency manager: NPM in Javascript and Composer in PHP for example.