This sometimes happen if you are going to install eaccelerator 0.9.5 for PHP optimizing. We just received our new dedicated server with debian sarge for our vBulletin forum and I’m optimizing it at the moment. There are many results in google for this error, however, none of them helped me.
What actually did help me was this:
apt-get install php5-dev
hth, one of my few non-windows posts 😉