We use PHP because we started as a startup and this was the fastest way to prototype our ideas back when we started. Today we are migrating away from most of it by re-implementing our frontend in React. We will most likely still keep the PHP core as our API because the database and the ORM we use is implemented very clean.
We use Kubernetes because we decided to migrate to a hosted cluster (not AWS) and still be able to scale our clusters up and down depending on load. By wrapping it with OpenShift we are now able to easily adapt to demand but also able to separate concerns into separate Pods depending on use-cases we have.