I spoke too fast. `headers_lists()` is not available in CLI mode, that means Unit Tests cannot success. I see three options here:

* Introduce dependency on XDebug for unit tests and use: `xdebug_get_headers()`
* Still use `header_lists`, and heavily modify behavior for Unit Test via `FirePHP_Test_Class` overriding. That would basically be the same "local cache" concept as currently suggested but only for unit test
* Keep the "local cache" as currently suggested

What would be the best option according to you ?