Change your highlight to "lua" instead of the default (highlight=lua). The coloring for the PHP is terrible.
Are you asking how to do it, what's wrong with your code (it does have an error I noticed) or just a guide? Lack of ending punctuation in your subject leaves this question.
As far as the error, you're missing brackets around your UnregisterEvent param.
Code:
self:UnregisterEvent("ADDON_LOADED")