Description:
------------
serialized string returned by serialize, contains invalid chars only when serializing an object with protected or private properties.

Reproduce code:
---------------
<?

class Foo {
	protected $bar = 123;
	//public $bar = 123;
}

$f = new Foo();

echo serialize($f);

?>

Expected result:
----------------
comment protected and uncomment public and the right result is given:

[[O:3:"Foo":1:{s:3:"bar";i:123;}]]

Actual result:
--------------
[[O:3:"Foo":1:{s:6:"
wich means:
[[O:3:"Foo":1:{s:6:"NULL*NULLbar";i:123;}]]
where NULL is the char \0