gistfile1.php

<?php
	set_error_handler('my_error_handler');

	session_start();

	class myclass implements Serializable
	{
		private $a = 1;
		private $b = 2;

		public function serialize()
		{
			return serialize($this->a);
		}

		public function unserialize($data)
		{
			$this->a = unserialize($data);
		}
	}

	function my_error_handler($code, $error, $file = NULL, $line = NULL)
	{
		throw new ErrorException($error, $code, 0, $file, $line);
	}

	$obj = new myclass();

	$_SESSION['obj'] = serialize($obj);

	$a->b();

// __sleep/__wakeup
// obj|O:7:"myclass":2:{s:10:"myclassa";i:1;s:10:"myclassb";i:2;} (Fatal error)
// obj|O:7:"myclass":1:{s:10:"myclassa";i:1;} (No fatal error)

// Serializable
// obj|s:22:"C:7:"myclass":4:{i:1;}"; (No fatal error)
// obj|s:22:"C:7:"myclass":4:{i:1;}"; (Fatal error)