I’m convinced that in the modern world of software engineering it is critically important to be comfortable building systems that use different programming languages and runtime environments for their various parts.

With the wide variety of options that are available and the capabilities of different languages and tools to make specific tasks simpler, being able to write PHP for some middle-ware pieces with Java or C# microservices on the back-end (perhaps in docker containers) and a javascript/typescript web UI is likely to allow your team to build faster and create a more capable system than keeping everything in a single language would have.

The wider the range of tools that are available to your team (and that your team is comfortable working with) the more flexibility you’ll have to move quickly and adapt to changing needs.