Hi all,
I have pushed into my repo the code for spatial index support in phpMyAdmin. Spatial indexes are allowed only for a spatial columns, so with the new feature the option is available only for spatial columns. Further, when editing, restrictions are placed not to let the user define spatial indexes on multiple columns or specify size as it is also not allowed.
You can test the feature on the demo server[1] and you may use 'opengis' database which has spatial data.