As far as I know, only PHP can't be used for this.
But since not many providers allow installation of Perl/Python/... scripts on shared, I'm wondering whether there is free solution for either
creation of thumbnails or full-size capturing on the fly / on demand and save it to server (since snapshot lets you only to show thumbnails on hover) - service
or
Flex/Flash solution to capture website and PHP to save it (or save it right with flex/flash) - code to run on server
Is it possible?
To capture how a website looks like, you first need somebody to render it.
Because you are usually optimizing a web site to run on the major browsers, you will want one of them to handle the rendering.
This (opening a browser instance, opening a certain web page, rendering it and dumping a screen shot of the results) is possible - it's how services like browsershots.org work.
It's just not trivial to set up, and requires total freedom in setting up the server (i.e. administrator privileges to install programs, set rights, etc.). It definitely is not possible to do with pure PHP, Perl, Python, or any other scripting language, on a restricted shared hosting environment.
If you're on Windows, the answer to this question may be of help.
For a list of snap shot services, see this question.