When you serialize and unserialize a PHP object, PHP will automatically check to see if there is a __sleep or __wakeup magic method in the class (depending on weather your serializing or unserializing).
When you serialize an object, PHP automatically checks to see if the object has a __sleep magic method. The sleep method is used because you might not want to serialize the whole object, sometimes just a specific array for example.
The example below demonstrates a class that would use the __sleep method when serialized:
The above code will save a serialized version of the 'info' array in the variables 'data'.