Serializing an object means converting it to a bytestream representation that can be stored in a file. This is useful for persistent data; for example, PHP sessions automatically save and restore objects. Serialization in PHP is mostly automatic—it requires little extra work from you, beyond calling the serialize() and unserialize() functions:
$encoded = serialize(something);
$something = unserialize(encoded);
Serialization is most commonly used with PHP’s sessions, which handle the serialization for you. All you need to do is tell PHP which variables to keep track of, and they’re automatically preserved between visits to pages on your site. However, sessions are not the only use of serialization—if you want to implement your own form of persistent objects, serialize() and unserialize() are a natural choice.
An object’s class must be defined before unserialization can occur. Attempting to unserialize an object whose class is not yet defined puts the object into stdClass, which renders it almost useless. One practical consequence of this is that if you use PHP sessions to automatically serialize and unserialize objects, you must include the file containing the object’s class definition in every page on your site. For example, your pages might start like this:
include "object_definitions.php"; // load object definitions
session_start(); // load persistent variables
?>
<html>...
PHP has two hooks for objects during the serialization and unserialization process: __sleep() and __wakeup(). These methods are used to notify objects that they’re being serialized or unserialized. Objects can be serialized if they do not have these methods; however, they won’t be notified about the process.
The __sleep() method is called on an object just before serialization; it can perform any cleanup necessary to preserve the object’s state, such as closing database connections, writing out unsaved persistent data, and so on. It should return an array containing the names of the data members that need to be written into the bytestream. If you return an empty array, no data is written
Conversely, the __wakeup() method is called on an object immediately after an object is created from a bytestream. The method can take any action it requires, such as reopening database connections and other initialization tasks.