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 4d76d8a0afdcc77aa838c79007ed7d304cad3188)