$ php -n -r '$d = new DOMDocument("1.0", "UTF-8"); $s = serialize($d); 
var_dump(unserialize($s)->encoding, $d->encoding);'

Warning: Unknown: Invalid State Error in Command line code on line 1
NULL
string(5) "UTF-8"


See bug#30323 && 
http://svn.php.net/viewvc/phpdoc/en/trunk/reference/var/functions/seri
alize.xml?r1=164053&r2=172271

I guess the simplest way to fix this is to say something along the 
lines of "classes that do not implement Serializable or the 
__sleep()/__wakeUp() functions cannot be reliably serialized"?

It's not that simple. For example ArrayObject in 5.3.1 does implement Serializable, but it did not back in 5.2.12 (latest stable 5.2) - and yet 
serialize/unserialize worked on it quite well:

weirdan@virtual-debian:/home/sam/trunk$ sudo update-alternatives --set php /usr/bin/php5
update-alternatives: using /usr/bin/php5 to provide /usr/bin/php (php) in manual mode.

weirdan@virtual-debian:/home/sam/trunk$ php -r 'var_dump(phpversion());
$c = new ReflectionClass("ArrayObject");
var_dump("is internal", $c->isInternal());
$q = new ArrayObject(array(1,2,3));
$w = unserialize(serialize($q));
assert("\$q[1] == \$w[1]");
var_dump("past assert");
var_dump("has __sleep or __wakeup", $c->hasMethod("__sleep") or $c->hasMethod("__wakeup"));
var_dump("implements Serializable", $c->implementsInterface("Serializable"));'

string(8) "5.2.12-2"
string(11) "is internal"
bool(true)
string(11) "past assert"
string(23) "has __sleep or __wakeup"
bool(false)
string(23) "implements Serializable"
bool(false)
weirdan@virtual-debian:/home/sam/trunk$

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.