The function names __construct, __destruct (see Constructors and Destructors), __call, __get, __set, __isset, __unset (see Overloading), __sleep, __wakeup, __toString, __set_state, __clone and __autoload are magical in PHP classes. You cannot have functions with these names in any of your classes unless you want the magic functionality associated with them.
PHP reserves all function names starting with __ as magical. It is recommended that you do not use function names with __ in PHP unless you want some documented magic functionality.
serialize() checks if your class has a function with the magic name __sleep. If so, that function is executed 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. If the method doesn't return anything then NULL is serialized and E_NOTICE is issued.
The intended use of __sleep is to commit pending data or perform similar cleanup tasks. Also, the function is useful if you have very large objects which do not need to be saved completely.
The __sleep method should return the value to serialize (usually $this), otherwise NULL is serialized.
Conversely, unserialize() checks for the presence of a function with the magic name __wakeup. If present, this function can reconstruct any resources that the 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.
Example 19-27. Sleep and wakeup
The __toString method allows a class to decide how it will react when it is converted to a string.