DOMDocument doesn't have any properties thus none are stored in the searilzed string. unserializing the string creates a DOMDocument object perfectly fine:

  $k = unserialize('O:11:"DOMDocument":0:{}');
  var_dump($k);

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.

"It is not possible to serialize PHP built-in objects."

Curt, I think you are not right. You are right that properties of internal classes are stored (visible e.g. with Directory class) but methods are not reconstructed after unserializing with e.g. Warning: Couldn't fetch DOMDocument.

So serailizing built-in object is useless.