I currently use php "includes" and have found it very usefull. Question is, when SE's spider, do they see php code or what the php code outputs?

Main reason I ask, is I've heard people using php for inserting meta tags. I would guess this could be done easily wiht php include and echo'ing the meta info. Or is it done a different way?

So do ALL SE's spiders "see" php code or the output?
Thanks

ronnie