Php developers should be very comfortable in *nix systems regardless of the fact they shouldn't really be responsible for servers.
If a developer is not confident on the command line then I would worry a bit. I am always using it for git, command line tools, mysql, ftp, editing files, ssh via the terminal. I wouldn't be able to do my job without knowing how to work with these tools.
On the topic of developers managing servers; I have built perhaps 20 over the past 5 years but never for any large scale or business critical server deployments. I always have the feeling that I haven't locked it down enough or disabled the right settings etc (which in all likelihood, I haven't). Having a decent working knowledge of at least the stack you work with is very very useful. Configuring apache, php, mysql, redis, password protection etc has helped me out immeasurably over the past few years however I have never had a server admin whos sole task it is to enable me to work.