It was a pretty large codebase, but we did have enough resources to do the rewrite while maintaining the current version. Sort of. It was migrated incrementally

It was a pretty large codebase, but we did have enough resources to do the rewrite while maintaining the current version. Sort of. It was migrated incrementally, and basically whenever we needed to fix a bug or add a feature to something that was still in the PHP part, we would migrate that specific functionality over to the new language. It did take several years, but since it was done incrementally, we got value from it along the way.