The pre-MCR 'rvdiffto' API parameter allows to supply a revision ID, or the special strings cur, prev or next to revision-related API modules; this will be then resolved to a revision and other returned revisions will be diffed against it.
On PHP >= 8.0, this parameter is no longer working as expected because cur, prev, and next are no longer special-cased - they are treated instead as if the user passed an invalid revision ID.