- * @param callback $function_to_check optional. If specified, return the priority of that function on this hook or false if not attached.
- * @return int|boolean Optionally returns the priority on that hook for the specified function.
+ * @param callback $function_to_check optional.
+ * @return mixed If $function_to_check is omitted, returns boolean for whether the hook has anything registered.
+ * When checking a specific function, the priority of that hook is returned, or false if the function is not attached.
+ * When using the $function_to_check argument, this function may return a non-boolean value that evaluates to false
+ * (e.g.) 0, so use the === operator for testing the return value.