Playing with XML_Parser, I was discouraged by the fact that
xml_set_object() actually copies an object even if it's been passed by
reference. This is a PITA when you want tag handlers to store parsed data
into object's properties and use these properties afterwards. You have to
assign references to these propeties in order to access values out of
handler context. Here's an example script:

<?php

require_once('XML/Parser.php');


$global_accum = '';


class MyParser extends XML_Parser {

	var $accum;
	var $ref_accum;

	function MyParser()
	{
		global $global_accum;
		$this->ref_accum = &$global_accum;
		$this->XML_Parser('ISO-8859-1');
	}

	function startHandler($parser, $tag, $attrs)
	{
		$this->accum .= "<$tag>";
		$this->ref_accum .= "<$tag>";
	}

	function endHandler($parser, $tag)
	{
		$this->accum .= "</$tag>";
		$this->ref_accum .= "</$tag>";
	}
}


header('Content-Type: text/plain');


$parser = new MyParser();

$data = <<<EOD
<?xml version="1.0"?>
<!DOCTYPE foo SYSTEM "/dtd/foo.dtd">
<foo>
  <bar/>
</foo>
EOD;

$err = $parser->parseString($data, TRUE);

if (PEAR::isError($err)) {
	die($err->getMessage());
}

echo "accum: ", $parser->accum, "\n";
echo "ref_accum: ", $parser->ref_accum, "\n";

?>

This produces:

accum:   
ref_accum: <FOO><BAR></BAR></FOO>