What do you mean internal pages? I don't think you can. JS used to be able to do it, but they've stopped that stuff now coz it's a security problem. I don't think PHP could do it as you need a browser (client) to display a page. PHP is strictly server-side. You probably need a browser plug-in.
I don't know whether Java or Flash could achieve this, but I *assume* they'd still need to ask for permission before undertaking snapshot tasks. Dunno.
It is not possible by using PHP only. It is possible to call an external program from PHP to do it, but this is very dependent on your hosting machine. There are API's that do this for you, but none are free (if they are, then they are very restricted).