Facet API - Rewrite facet items via callback function
Make sure this filter is selected for your facet(s): "Rewrite facet items via callback function"
admin/config/search/apachesolr/settings > Facets > YOUR FACET > configure filters > [x] Rewrite facet items via callback function
You can verify your hook is implemented using drush and devel module:



drush hook facet_items_alter
Enter the number of the hook implementation you wish to view.
[0]  :  Cancel
[1]  :  my_module_name


If you see something similar to above, then your hook is implemented. To see the implementation code, enter a number (1 in this example).