You can run Mozilla server side (along with a ton of other browsers).
I'm sure if you just think about it, you could probably open up mozilla, feed it a URL, and do a screen capture.
I'm not too into Linux on the desktop, but I could probably bang it out in OSX within a day.
I really doubt you'll be using PHP for the task though, unless you're up for writing a parser/rendering engine in PHP, which would be a monumental undertaking. Maybe for doing a sys call to get things going, but that's about it.