In this blog entry Arnold Daniels talks about an issue he had in the past (needing a bit more functionality than the PEAR DB library could offer) and how he ended up solving it with what he calls a "dark corner" of PHP - class casting.
PHP has a function serialize, which can create a hash from any type of variable, scalars, array, but objects as well. Using the unserialize function, PHP can recreate the variable from the serialized hashed. If we look at how an object is serialized, we see only the properties and the class name are stored.
His method allows for class manipulation via changes to the serialized class information (like changing the value of the name parameter). His "casttoclass()" function makes changing this value simple.