Manual:$wgCategoryCollation

Details
A version indicator for collations that will be stored in cl_collation for all new rows.

Used when the collation algorithm changes: a script checks for all rows where cl_collation != $wgCategoryCollation and regenerates cl_sortkey based on the page name and cl_sortkey_prefix.

Currently only supports 'uppercase', which just uppercases the string. This is a dummy collation, to be replaced later by real ones.