PR Adding it to mongodb-constants (similar to what we'll do here, but for the mongo-php-library): https://github.com/mongodb-js/compass/pull/7048 

This ticket involves either making sure we get an update of the php definitions that have this $rankFusion stage (if that exist) or the ticket involves adding it ourselves to the php definitions in mongo-php-library

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Starting in 8.1, users will be able to use $rankFusion (FAQ) to fuse the results of agg stages like $vectorSearch and $search that yield ordered result sets into a deduplicated, ordered set.

We would like for this stage to appear in downstream UIs that allow users to test aggregation stages, like Compass and Data Explorer.

Note that until this is backported to 8.0, this should not be supported on 8.0.