Description:
------------
I have tried to json_encode a DOMDocument and does not return false and does not throw exception.

Instead just returns {}, entirely ignoring the contents of the DOMDocument it is meant to be decoding.

The problem for me here is that the json_encode logically "fails" but does not indicate this.

(In an ideal world I would expect it to decode the DOMDocument in exactly the same way as it does a SimpleXML document

Test script:
---------------
<?php

$a = new DOMDocument();
$a->appendChild($a->createElement("hello","there"));

print "Encode DOMDocument" . PHP_EOL;
if (($output = json_encode($a)) === false) {
  print "fails with error!"; 
} else {
  print $output;
}
print PHP_EOL . PHP_EOL;

print "Encode DOMDocument document element" . PHP_EOL;
if (($output = json_encode($a->documentElement)) === false) {
  print "fails with error!"; 
} else {
  print $output;
}
print PHP_EOL . PHP_EOL;


print "Encode SimpleXML equivalent" . PHP_EOL;
if (($output = json_encode(simplexml_import_dom($a->documentElement))) === false) {
  print "fails with error!"; 
} else {
  print $output;
}
print PHP_EOL . PHP_EOL;


Expected result:
----------------
I would expect either DOMDocument or DOMDocument document element to encode sensibly.

Or at the very least I would expect json_encode to return false or throw exception should the failure be considered catastrophic.


Actual result:
--------------
Encode DOMDocument
{}

Encode DOMDocument document element
{}

Encode SimpleXML equivalent
{"0":"there"}