Verifying XML Support

Before we get started, make sure that the computer you're working with was set up to support XML. If you are not sure that your existing PHP installation has XML support, check phpinfo.php from Chapter 2. If XML support is set to active, the output will contain a section similar to the following:

If you do not see the above, then don't worry. We will cover setting up XML support for each API in the appropriate section. For further information refer the PHP manual for instructions on installing PHP with XML support at: http://www.php.net/manual/en/ref.xml.php

XML APIs Comparison

The three main PHP APIs available for working with XML are SAX, DOM, and PRAX. SAX and DOM are available in most programming languages. PRAX is a much newer API and it is a port of the Perl XML::RAX API.

These XML APIs that are "native" to PHP have some problems:

• SAX has some problems that can cause a server crash.
• DOM has memory leaks in the code library libxml. The memory leaks are in all the PHP releases (current release is 4.0.6), on most platforms, Windows98/NT/2000, Linux (Redhat, Debian, Mandrake, SuSE), and xBSD.
• PRAX does not work very well with complex XML documents.

In spite of these minor difficulties, each of these APIs deals with XML in different ways, and provides different functions. Choosing which API to use depends on your application.

Let's now compare some of the more important parts of each API:

	SAX	DOM	PRAX
Model	Event-driven	Document tree	Recordset
Installation	Installed by default	Requires installation	Standalone class
Document Size	Small to huge	Small to medium	Small to medium
Code Complexity	Moderate to complex	Moderate to complex	Simple to moderate
Supports Read, Write, Change	Only read	Yes	Only read
Best Fit Application	Machine readable and generated XML	Complex XML document	Machine readable and generated XML
Maturity within PHP	Expat has been in PHP since Version 3.0 and it seems to be pretty mature and stable	Experimental	Very young
XML Format	XML 1.0, thus you can use it for SML too	XML 1.0, thus you can use it for SML too.	SML

SAX vs. DOM

The main difference between SAX and DOM is how they interact with an XML file. SAX reads the file in relatively small chunks, parses them, calls a handler, and performs a task. The task that is performed by the instantiated parser is up to the developer. Using SAX we can in theory parse an XML document of infinite length. In this chapter we will be looking at displaying XML as a table so we will set up the handlers to do this.

DOM loads the whole XML file into memory. Consequently it places a higher load on the server. Once the document is in memory we can interact with it in much the same way we would interact with a multi-dimensional array. We can add, read, and delete (unlink) nodes. This difference between APIs won't matter for small XML files and low volume servers, but when you get large XML documents you may notice a performance difference.

PRAX vs. SAX and DOM

With PRAX we can quickly parse an XML document with a minimum of code and complexity. Reading files with PRAX is programmatically easier than SAX, and doesn't have the memory overhead of DOM.

PRAX is a new API and is relatively unknown. In its current state I don't recommend using it for commercial or production use. PRAX is here to provide another example of working with XML and PHP. We shall go into depth for each one further in this chapter where we'll discuss each API and look at the PHP code that will render an XML document into HTML.

SML
Professional PHP4 Programming
The SAX Model