RESTBase is making sure that we have a unique X-Request-ID header on each request, and sets this on each outgoing request. Other services are now starting to forward this header in their own outgoing requests as well (built into the service template, T97207 for parsoid).
To leverage this header to trace incoming requests to the PHP API back to their origin(s), the PHP API should log the X-Request-ID header along with other log data. Since internal requests to the PHP API typically don't pass through Varnish, it probably makes sense to tackle this at the API (Apache?) layer first.