Nuances of serialization

I think many people know the "magic" methods __ sleep and __ wakeup to control the serialization of objects.

It would seem simple:
__sleep is called before the serialization so that you can remove unnecessary connections (DB, MemCash, etc.).
__wakeup is called after unserialize, so that the necessary connections can be automatically returned to the backside.

But there is a nuance. And I came across him 4 times in the last 5 years of work.

1. The __sleep method must return an array with a list of properties that need to be serialized.
2. If the object is serialized and then continues to be used, it may be corrupted, since the __sleep method was called during serialization.
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The example from the manual misleads

public function __sleep ()
{
mysql_close ($ this-> link);
}

Having done everything following the example, not only will the result of serialization be NULL, but the object itself will be corrupted)))

So be careful and carefully read the manual))))