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AFAIK the sites like browsercam and the one that show in Safari all have to bring the page up on an actual client and then do an actual screen capture. I'm not sure what language the brograms are written in, but I think it's safe to say it's not PHP.
I wonder if instead of thumbnails of a website, you could infact shrink the webpage (using iframes?)and then just display it as a thumbnail on a page with others?
That would create a unique toy by which people could see multiple websites and click on the small one to open it out.