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PHP and XML: Using the expat functions

by: Justin Grant
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July 30, 2000

The Extensible Markup Language is definitely something that most developers will want to add to their toolbox. XML is a W3C standard that is open, language neutral, API-neutral, streamable, textual, human-readable and is a way of bringing structured data to the web. XML is a subset of SGML and is not a markup language in itself but allows the author to define their own markup language where it is best suited for representing hierachical data. Now, parsing XML documents with PHP is not a topic that has been covered extensively as far as I have seen on the web and elsewhere. There is some very useful information in the PHP manual on XML parsing functions but this seems to be all the information that I could find. Other languages seem to have much more information and working examples of using XML than PHP does, so in this article I will attempt to do my part to change this.
I will walk the reader through a fairly simple application of XML that I used to implement a news system for my home page. I actually use this on my web site and it works really well. Feel free to use it on yours if you like. Ok, let's get started !
To use the XML parsing functions available to PHP you will need to have a module that supports XML installed on your web server. This means you will probably have to recompile your module to support XML, please look here for more info on how to do this. The XML parsing functions are really wrappers around the expat parser which is a SAX parser, which means Simple API for XML. The other kind of parser is a DOM parser, easier to use, an example of this is Microsoft's MSXML component which lets the programmer navigate through a tree style object accessing nodes and elements. The way the expat parser (or any SAX parser) allows you to parse an XML document is to specify callback functions for different tag types in the XML document being parsed. When the parser starts parsing your XML document and encounters a tag it will call your function and process the tag as specified in your function before continuing. You could look at this as an event driven way of parsing.

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