an XML validator for Zend_Validate, because... there isn't one

Zend_Validate_Xml.php

<?php

/**
 * @see Zend_Validate_Abstract
 */
#require_once 'Zend/Validate/Abstract.php';

/**
 * @category   Zend
 * @package    Zend_Validate
 * @copyright  Copyright (c) 2012 Lee Bolding
 * @author Lee Bolding <lee@leesbian.net>
 * @license    http://framework.zend.com/license/new-bsd     New BSD License
 */
class Zend_Validate_Xml extends Zend_Validate_Abstract
{
    const XML = 'xml';
    const XML_VERSION_1 = '1.0';
    const UTF_8 = 'UTF-8';
    
    const RNG = 'rng';
    const XSD = 'xsd';
    const DTD = 'dtd';

    /**
     * @var array
     */
    protected $_messageTemplates = array(
        self::XML => "%message%"
    );

    /**
     * @var array
     */
    protected $_messageVariables = array();

    /**
     * DomDocument
     *
     * @var mixed
     */
    protected $_doc;
    
    /**
     * validation type value
     *
     * @var mixed
     */
    protected $_type;
    
    /**
     * validation schema value
     *
     * @var mixed
     */
    protected $_schema;

    /**
     * Sets validator options
     *
     * @param  mixed|Zend_Config $args
     * @return void
     */
    public function __construct($args)
    {
      $this->_doc = new DOMDocument(self::XML_VERSION_1, self::UTF_8);
      
        if ($args instanceof Zend_Config) {
            $args = $args->toArray();
        }

        if (is_array($args)) {
            if (array_key_exists('type', $args))
            {
                $type = $args['type'];
            }
            
            if ($type && array_key_exists('schema', $args))
            {
                $schema = $args['schema'];
            } else {
                #require_once 'Zend/Validate/Exception.php';
                throw new Zend_Validate_Exception("Missing option: 'schema'");
            }
        }

        $this->setType($type);
        $this->setSchema($schema);
    }

    /**
     * Returns the type option
     *
     * @return mixed
     */
    public function getType()
    {
        return $this->_type;
    }

    /**
     * Sets the type option
     *
     * @param  mixed $type
     * @return Zend_Validate_Xml Provides a fluent interface
     */
    public function setType($type)
    {
        $this->_type = $type;
        
        return $this;
    }
    
    /**
     * Returns the schema option
     *
     * @return mixed
     */
    public function getSchema()
    {
        return $this->_schema;
    }

    /**
     * Sets the schema option
     *
     * @param string $schema The validation schema - AS A STRING
     * @return Zend_Validate_Xml Provides a fluent interface
     */
    public function setSchema($schema)
    {
        $this->_schema = $schema;
        
        return $this;
    }

    /**
     * Defined by Zend_Validate_Interface
     *
     * Returns true if and only if $value is valid XML
     *
     * @param  mixed $value
     * @return boolean
     */
    public function isValid($value)
    {
        libxml_use_internal_errors(true);
        
        $this->_setValue($value);
        
        // Load the xml document in the DOMDocument object
        $this->_doc->LoadXML($value);
        
        // switch validation based on specified validation type
        switch ($this->_type)
        {
          case self::XSD:
            $this->_doc->schemaValidateSource($this->_schema);
          break;
        
          case self::RNG:
            $this->_doc->relaxNGValidateSource($this->_schema);            
          break;
        
          default: //DTD
            $this->_doc->validate();            
          break;
        }
            
        $errors = libxml_get_errors();
    
        if (empty($errors))
        {
          return true;
        }

        $error = $errors[0];
        if ($error->level < 3)
        {
          return true;
        }
        
        $lines = explode("r", $value);
        $line = $lines[($error->line)-1];

        $this->_messageTemplates[self::XML] = 'Line:' . $error->line . ' ' . $error->message;
        
        $this->_error(self::XML);
        
        return false;
    }
}