Suppress UnusedFunctionParameter with underscore prefix

In cases where you need some of the parameters, but not all of them, for example:

// @codingStandardsIgnoreLine -- Not using the $slug parameter.
public function stageAction($id, $slug, $stageId)

TypeScript allows you to use an underscore prefix to suppress the error/warning, like this:

public stageAction(id, _slug, stageId)

Or, in PHP:

public function stageAction($id, $_slug, $stageId)

It's a lot cleaner and less verbose in my opinion.

The variable name sniff would need an option to allow a single underscore in front of function parameters.