Add default php entry point

The Wikimedia Jenkins jobs expect MediaWiki extensions to have a default
entry point for the extension.  That greatly simplify loading extensions
automatically.

Change-Id: I51ab893d51d0f1c8002a3ccf7d2cfcdf2f017966
(cherry picked from commit 0ca29af9a2088d00af557fe09453d74ec94db0fa)