The PHP script single_book.php, is the server side portion of the Ajax application. It receives two variables in the query string: the isbn number and a boolean variable indicating whether the browser sending the request supports XSLT. The script either outputs XML or HTML depending upon the value of the xslt variable.

PHP - single_book.php:
<?php
if (@$_GET['source']) {
    highlight_file(__FILE__);
    exit;
}
    /* load external variables */
    $isbn = htmlentities(@$_GET['isbn']);
    $xslt = (boolean) @$_GET['xslt'];

    /* this DOMDocument will contain the XML for a single book */
    $returnDom = new DOMDocument;

    /* load the complete libarary XML */
    $xml = new DOMDocument('1.0');
    $xml->load('xml/library.xml');

    /* use an xPath expression to extract the book containing the isbn number we want */
    $xPath = new DOMXpath($xml);
    $book = $xPath->evaluate("/library/books/book[@isbn='$isbn']")->item(0);

    /* if the book was found, append it to our return DOM object */
    if ($book) {
        $book = $returnDom->importNode($book, true);
        $returnDom->appendChild($book);
    }

    if ($xslt) { // browser supports XSLT, let it do the transformation
        header('Content-Type: application/xml'); // send the correct MIME type
        echo($returnDom->saveXml());
    } else {// borwser does not support XSLT, therefore we do the transformation on its behalf
        /* load the stylesheet */
        $xsl = new DOMDocument;
        $xsl->load('xml/single_book_html.xsl');
        
        /* import the stylesheet */
        $processor = new XSLTProcessor;
        $processor->importStyleSheet($xsl);
            
        /* transform it to HTML and output the result */
        echo($processor->transformToXml($returnDom));
    }
?> 



Due to the different implementations of XSLT, the Ajax engine is the most complex part of the application. Several helper functions have been defined, which are as follows. Each function chooses the browser-dependent method to complete the task.

• createDomFromUri(uri, freeThreaded) – loads and returns an instance of a DOMDocument from a specified location on the Internet. n.b: only documents from the same domain can be loaded
  
  freeThreaded - (Internet Explorer Only) If set to true a FreeThreadedDomDocument is returned.
  
• getDom(freeThreaded) – returns an instance of  DOMDocument object.
  
  freeThreaded - (Internet Explorer Only) If set to true a FreeThreadedDomDocument is returned.
  
• getXMLHTTP() - returns an instance of an XMLHTTPRequest object. Returns false if it is unavailable.
  
• loadXmlFromString(sXml) - loads an XML DOMDocument object from a valid string representation of XML.
  
  sXML - the string representation of the XML to be parsed
  
• loadXslStylesheet(uri) – loads an XSLT stylesheet returning an XSLTProcessor object
  
   uri – the location of the XSLT stylesheet
• parseXmlDom(oXmlDom) – returns true if the Xml document is valid and well formed and false if not
  
  oXmlDom – the DOMDocument to check
  
• transformXml(oXmlDom, oXsltProcessor) – transforms an XML DOMDocument. W3c compliant browsers return a DOMDocument object representing the transformed XML. Internet Explorer returns a string representation of the transformed XML.
  
  oXmlDom – the XML DOMDocument object to be transformed
  oXsltProcessor – the XSLT processor object to use when making the transformation.
  

The code from these functions is declared in the xml_helper_functions.js file contained in the ZIP file accompanying this article.