The file is reloaded at each request. (That may or may not mean it's loaded from disk.) Installing something like APC [mostly] ensures you don't hit the disk each time.

$databaseConfig is rebuilt with each requests. If it comes with notable overhead, you can also use something like APC to cache it in memory.

The file is reloaded and re-parsed at each request. (That may or may not mean it's loaded from disk.) Installing something like APC generally ensures you don't hit the disk or take the parsing hit each time.

The code is re-executed entirely with each request, even if the bytecodes are cached. So, any variable, including $databaseConfig, is rebuilt with each request. If it comes with notable overhead, you can also use something like APC to cache it in memory.