You can, as described in other answers, do it with nasty black magic PECL extensions.
Though, you seriously don't want it. Any problem you want to solve in OOP there's an OOP-compliant way to do it.
Runtime type hierarchy modifications are not OOP-compliant (in fact, this is consciously avoided). There are design patterns that should fit what you want.
Please, tell us why do you want that, I'm sure there must be better ways to do it ;)
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