Each SkillSet should provide his own skills and should not share the
skills with other skill sets.
Therefore use an instance property instead of static.
Allow to retrieve skill and skill set by ID.
Returns PHP classes with proper getter for most important info.
All info are available via PHP array from those classes.
Link looks more like a service.
When dealing with dependency injection it would make sense to create a
single instance with settings.
Following calls can then ask to create a button or link with all
necessary information.