While more and more hosting environments are adding PHP 5 support, many of them
are still doing weird things like having PHP 4 for '.php' files and PHP 5 only
for '.php5' files.
There are some third-party directions for poking about and renaming files in MW
to work this way, but it's kind of lame to have to do that.
The installer should be able to detect it's running on PHP 4 and suggest trying
an alternate '.php5' file, which would basically just consist of an include().
These would only have to be provided for a few key entry points:
config/index.php index.php redirect.php thumb.php img_auth.php api.php
Other files are included indirectly, so their extensions don't really matter.
Additional config var or vars would be required to make sure the URLs are correct
for things other than the main $wgScript, but that shouldn't be very difficult.