By default PHP provides shared-nothing environments to ensure that whatever happens to PHP's state in one request has no effect on other requests, so all function tables are cleaned up, all file handles are closed etc. In a few rare cases this is not what people like, for that PHP introduced "persistent connections" of different kinds. Testing those is a bit annoying as you have to configure a webserver and ensure to hit the same instance over the course of a test and then use a load generator, probably one which can detect a failure. Additionally by having a webserver in the game there is more code being executed, which might mean an additional source for trouble while debugging. An alternative might be using FastCGI, while that adds it's own issues for such a test.
To solve this for myself I, some time ago, wrote a PHP SAPI module called pconn and pushed it to github. (A SAPI is the component in PHP which implements the communication with the web server or whatever triggers PHP requests) The general idea was to have a lightweight SAPI which does nothing but emulate a bunch of requests. I had it some where on my list of things to blog about, but well, low prio.
Now some time later it seems like Derick was doing some stuff with persistent connections, too, and figured that the new embedded web server is a good thing for such tests, too. While he didn't know about my solution, as one could see in a short discussion on twitter we had:
In other news, the new CLI web server in PHP 5.4 is brilliant for debugging issues with extensions that span more than one request.— Derick Rethans (@derickr) March 15, 2012
@phperror: You need to write about that stuff :-ž— Derick Rethans (@derickr) March 15, 2012
Now I've contradicted myself: Above I was proudly writing about this being lightweight and easy to debug, but in the tweet I mentioned threading. And well threading always includes lots of trouble to code. But yeah, over time I figured out that this was a good foundation to solve a second issue which has has to be done for PHP: PHP can be run in threaded environments, which in general is not advised. When doing that the old party rule applies: What happens in a thread stays in thread. Different threads should not impact the requests handled in other threads. Now testing for race conditions is even harder than testing persistent connections and additional web server code hurts even more. So my little SAPI became a lot bigger and can now be compiled in two modes. Either simple and short in non threaded mode or with all the extra stuff in threaded-mode which will allow running PHP requests in parallel threads in loops.
In case you find yourself working on some PHP extensions where this might help: Check the github repository and the README and drop me a line if anything is unclear.