php-parser/lib/PHPParser/Serializer/XML.php

<?php

class PHPParser_Serializer_XML implements PHPParser_Serializer
{
    protected $writer;

    /**
     * Constructs a XML serializer.
     */
    public function __construct() {
        $this->writer = new XMLWriter;
        $this->writer->openMemory();
        $this->writer->setIndent(true);
    }

    public function serialize(array $nodes) {
        $this->writer->flush();
        $this->writer->startDocument('1.0', 'UTF-8');

        $this->writer->startElement('AST');
        $this->writer->writeAttribute('xmlns:node',    'http://nikic.github.com/PHPParser/XML/node');
        $this->writer->writeAttribute('xmlns:subNode', 'http://nikic.github.com/PHPParser/XML/subNode');
        $this->writer->writeAttribute('xmlns:scalar',  'http://nikic.github.com/PHPParser/XML/scalar');

        $this->_serialize($nodes);

        $this->writer->endElement();

        return $this->writer->outputMemory();
    }

    public function _serialize($node) {
         if ($node instanceof PHPParser_Node) {
            $this->writer->startElement('node:' . $node->getType());

            if (-1 !== $line = $node->getLine()) {
                $this->writer->writeAttribute('line', $line);
            }

            if (null !== $docComment = $node->getDocComment()) {
                $this->writer->writeAttribute('docComment', $docComment);
            }

            foreach ($node as $name => $subNode) {
                $this->writer->startElement('subNode:' . $name);
                $this->_serialize($subNode);

                $this->writer->endElement();
            }

            $this->writer->endElement();
        } elseif (is_array($node)) {
            $this->writer->startElement('scalar:array');
            foreach ($node as $subNode) {
                $this->_serialize($subNode);
            }
            $this->writer->endElement();
        } elseif (is_string($node)) {
            $this->writer->writeElement('scalar:string', $node);
        } elseif (is_int($node)) {
            $this->writer->writeElement('scalar:int', $node);
        } elseif (is_float($node)) {
            $this->writer->writeElement('scalar:float', $node);
        } elseif (true === $node) {
            $this->writer->writeElement('scalar:true');
        } elseif (false === $node) {
            $this->writer->writeElement('scalar:false');
        } elseif (null === $node) {
            $this->writer->writeElement('scalar:null');
        } else {
            throw new InvalidArgumentException('Unexpected node type');
        }
    }
}