[Tim]

do
   for _, frame in pairs({
      "PlayerFrame",
      "TargetFrame", -- you can keep adding more frames
   }) do
      frame:SetAlpha(0)
      frame:HookScript("OnEnter", function(self) self:SetAlpha(1) end)
      frame:HookScript("OnLeave", function(self) self:SetAlpha(0) end)
   end
end