The requested URL /selenium-server/core/RemoteRunner.html was not found on this server.

So you’ve just installed Selenium server and you want to run a phpUnit test.
You’ve started the server:

java -jar selenium-server-standalone-2.6.0.jar

And now you run your phpUnit:

phpunit tests/MyTest.php

And surprisingly you get this error in your browser:

The requested URL /selenium-server/core/RemoteRunner.html was not found on this server.

What to do?
Here is a solution that helped me:
1. Open your browser (the one that is started by Selenium);
2. Go to proxy settings (For firefox 6 on linux: Edit->Preferences->Advanced->Network->Connection: Settings->Manual Proxy configuration and set there host: localhost and port: 4444) and change the host to localhost and port to 4444;
3. Rerun your tests

Note: In case your firefox doesn’t start, modify your phpunit.xml file and set

 browser name=”Firefox” browser=”*custom /bin/firefox”

Or depending where your firefox is.
Good luck!