AJAX and PHP

Chapter 2
}
}
else
{
// display status message
alert("There was a problem retrieving the data:\n" +
xmlHttp.statusText);
}
Apart from the error-handling bits, it's good to notice the xmlHttp.responseText method that
reads the response from the server. This method has a bigger brother called
xmlHttp.responseXml, which can be used when the response from the server is in XML format.
Unless the responseXml method of the XMLHttpRequest object is used, there's really no
XML appearing anywhere, except for the name of that object (the exercise you have just
completed is a perfect example of this). A better name for the object would have been
"HttpRequest". The XML prefix was probably added by Microsoft because it sounded
good at that moment, when XML was a big buzzword as AJAX is nowadays. Don't be
surprised if you will see objects called AjaxRequest (or similar) in the days to come.
Working with XML Structures
XML documents are similar to HTML documents in that they are text-based, and contain
hierarchies of elements. In the last few years, XML has become very popular for packaging and
delivering all kinds of data.
Incidentally, XML puts the X in AJAX, and the prefix in XMLHttpRequest. However, once again,
note that using XML is optional. In the previous exercise, you created a simple application that
made an asynchronous call to the server, just to receive a text document; no XML was involved.
XML is a vast subject, with many complementary technologies. You will hear people
talking about DTDs, schemas and namespaces, XSLT and XPath, XLink and XPointer,
and more. In this book we will mostly use XML for transmitting simple structures of
data. For a quick-start introduction to XML we recommend http://www.xmlnews.org/
docs/xml-basics.html. If you don't mind the ads, http://www.w3schools.com/
xml/default.asp is a good resource as well. Appendix C available at
http://ajaxphp.packtpub.com contains an introduction to XSLT and Xpath.
You can use the DOM to manipulate XML files just as you did for manipulating HTML files. The
following exercise is similar to the previous exercise in that you read a static file from the server.
The novelty is that the file is XML, and we read it using the DOM.
Time for Action—Making Asynchronous Calls with XMLHttpRequest
and XML
1. In the foundations folder create a subfolder called xml.
2. In the xml folder, create a file called books.xml, which will contain the XML structure
that we will read using JavaScript's DOM. Add the following content to the file:
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www.it-ebooks.info