The magic functions __sleep and __wakeup

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# The magic functions `__sleep` and `__wakeup`

serialize() checks if your class has a function with the magic name `__sleep`. If so, that function is being run prior to any serialization. It can clean up the object and is supposed to return an array with the names of all variables of that object that should be serialized.

The intended use of `__sleep` is to close any database connections that object may have, committing pending data or perform similar cleanup tasks. Also, the function is useful if you have very large objects which need not be saved completely.

Conversely, unserialize() checks for the presence of a function with the magic name `__wakeup`. If present, this function can reconstruct any resources that object may have.

The intended use of `__wakeup` is to reestablish any database connections that may have been lost during serialization and perform other reinitialization tasks.

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