At the moment you cannot generate GIF's with the latest versions of the image function. As for animated PNGs, I belive they're called MNGs and are not yet supported by anything on the web. There is an alternative, however, involving Flash generation. It's not exactly what you're looking for, but PHP 4 has a robust Flash generation function called SWF and another 3rd party called Ming. I've no experience with either, but they're your best bet.
RCM
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