* Removing unused local vars.
* Removing unused global declarations.
* removing one or two pass-by-refs parameter declarations where not required by PHP5.
* Adding some local variable declarations / initializations (initialized to either to array(), null) to keep static code analyzer happy.
* Changing lines like this: "extract( $dbw->tableNames( 'page', 'archive' ) );" to be like this: "list ($page, $archive) = $dbw->tableNamesN( 'page', 'archive' );".
* Add one or two braces to if-else blocks.
Caches for wikis with table prefixes will be lost on upgrade, caches for wikis without table prefixes will be preserved. Custom cache keys in extensions can be migrated at leisure. Extensions which write to core cache keys should be migrated ASAP, as I have done with Special:Makesysop.