If you’re working on a theme or a plugin for WordPress and you want to highlight an active submenu item, then your implementation is going to vary based on where you want to highlight the actual item.

An overexaggerated menu to help drive this point home.
This is one of those times where it’s helpful to have clear terminology for what you’re trying to modify:
- Are you working on trying to highlight an active submenu in the admin menu,
- Or are you working to highlight an active submenu on the front-end of the theme?
There’s no consistent way to do this. For what it’s worth, I don’t think they should be as they are two completely different entities (for lack of a better term). Perhaps having some semi-consistent filter names would be nice, but that’s about it.
Regardless, when you set out to highlight an active submenu item, it’s important to note which part of the project you’re working on and then go from there.
