To expand on bholbrook's comment, if you receive this: "Warning: domdocument::domdocument() expects at least 1 parameter", it is due to the old domxml extension, which you need to disable.
domxml overwrites DOMDocument::_construct with an alias to domxml_open_mem, so this code:
<?php
$doc = new DOMDocument();
?>
...essentially does this:
<?php
$dom = domxml_open_mem();
?>
...which is why PHP complains about expecting at least 1 parameter (it expects a string of XML).