Phillips head - sometimes even the impact driver will round them out, as mine did.
Couple suggestions, make sure you have the right bit and you might want to try a touch of valve lapping compound on it to help it "bite"
But, sometimes they strip, and here is one way that I have used successfully over the years.
Tools: A hammer and a pointed drift
I use the drift to tap the screw loose - works great most of the time!
When it fails, there are other options -
Here was a "difficult" cam bolt on a Porsche 944 -
Sometimes you drill the hole to the proper size, in this case, is was an allen head, so it self-rounded itself to the proper size.
Sometimes, well, sometimes - you get it out -
But it was a little more than you bargained for.....
It's nice when you can tap tap tap 'em out.......