RE: [PHP] Re: XML, what is that supposed to do?

uff... that was long but very useful, thanks Terry - a good contribution for our developers team in here whom I just forwarded your email and for PHP-General archives. Cheers, Maxim Maletsky -----Original Message----- From: Terrence Chay [mailto:tychay@qixo.com] Sent: Wednesday, January 17, 2001 9:41 AM To: PHP User Group Subject: [PHP] Re: XML, what is that supposed to do? on 1/16/00 9:38 AM, Brandon Orther at brandon@webintellects.com wrote: > I have seen a lot of people ask for XML support for PHP. I was wondering > what it does that makes it good for PHP. That's a hard one to answer--I'll try anyway. I'm sorry if it sounds a bit simple-minded but that's the sort of person I am ;-). XML stands for eXtensible Markup Language. It's an attempt by both large corporations (notably Microsoft, Oracle, and IBM) as well as the standards organizations (notable the W3C) to create a "lingua franca" for the web. A lot of understanding it has first to have a slightly different perspective of what we mean by "the internet" and "the web". First when most people say "the internet" now, they pretty much mean "the web" and e-mail. When people say "the web" they mean HTTP and HTML (with a little SSL thrown in for e-commerce). HTTP is the transport protocol (how it is delivered) and HTML is the markup language (the message). XML attempts to replace and supersede HTML without saying anything about HTTP (though one can assume that most of the delivery will be done via HTTP, much to the chagrin of many security administrators who depend on firewalls). XML is a markup language like HTML. Unlike HTML, the markup language is extensible (basically think of it as saying you can define your own tags and attributes). This means you can make descriptive tags such as <book type="paperback"><AUTHOR>Joe Blogs</AUTHOR><TITLE>SATs - How to be beaten by the system</TITLE><SUBJECT>Test preparation</SUBJECT></book> Which looks a lot like HTML but isn't. Interestingly, the tags are descriptive of the content which beats the hell out of UN/EDIFACT if you've ever had to do any work for big business. Other differences are the rules are more rigid than HTML: all tags must close, all attributes must be quoted, all reserved characters must be escaped properly, all tags and attributes are case sensitive. The default format for display is double byte encoded characters (UTF-16 / UNICODE) (Note: The default used by PHP seems to be UTF-8 so you should change it to that charset in the XML directives line).- So basically what you have when you are done is a text based hierarchical data structure that's extensible and machine readable. That's all XML is. Now the things you can do with it. Obviously for one I can use this to serialize objects in PHP very easily since I can store objects in XML representation which is just a string to be saved. The WDDX module does that in some standard way. A note about standards. Since XML is extensible, there is a need to be specified so that I can communicate with you and we understand each other. XML is really more like a markup language FORMAT than a language (or seen another way, it's a standard but not a specification). There are various specifications and attempts at specifications out there and are usually referred to as DTDs, Document Type Definitions, or Schema. It used to be you specified your Schema in another markup language called SGML but then some people figured if XML is so extensible you should be able to specify your own Schema in an Schema language which itself is XML. This is known, not surprisingly, as XML Schema. Which represents another thing you can do with XML: Use XML to specify XML data formats. A useful one for web programmers right now is you can use XML to turn XML into other XML formats. This is done through XSL-T (eXtensible Stylesheet Language - Transform) which is built into a PHP module called Sablotron (Side Note: I couldn't compile Sablotron 0.50 in PHP yet, it failes during the linking step in Apache and claims that it can't find some library that is in Expat). Sablotron (and many XML-T parsers) is a little robust in that you can use it to transform it into HTML and text too. This warrants a bit larger description... Basically XSL works by taking an input XML file (we'll call this the "data store") and using another XML file written using the XSL specification (we'll call this the "rules file") to create another file in a different XML format (we'll all this the "presentation file"). Obviously when the presentation file is in XML, we can chain another rules file to it to make another presentation file and so on. XSLT parsers such as Sablotron allow us to do just that. Why is this powerful? The best way is through examples (1) Our company builds a search engine that goes out and does a real-time travel comparison engine of 25 separate travel websites. Given that each search does this, we offload this to a business rules server that creates this and returns the results. Because we add sites and features almost at will, this messaging standard had to be extensible. The webserver has to communicate with this business rules server and understand it. A stylesheet can ensure that the message that gets sent to the web server is always in line with what the webserver can understand even if we upgrade our features on the business rules server. (2) Furthermore, we have some nasty internal business rules embeded in our XML data store on the business rules server. An XSLT filter allows us to remove these internal business rules before delivery. This makes our business objects resellable to third parties as an application service without compromising our internal ones and requiring much coding. we can use the same XML data store to store private and public information. (3) The webserver itself needs to parse and deliver the data. That data may vary on our site vs. a cobranded site. With XSLT you can transform XML on the fly to XHTML (a superset of HTML) and tack on your presentation layer (nice little font tags and setting the color and whatnot). A different XSLT for a different browser or cobrand, yet the same datastore for all of them. This is called "separating your presentation from your data". Microsoft calls this 3-tiering, n-tiering, DNA, NetDocs, and now dotNet. (Well some of the later ones are a bit more than just 3-tiering, but the basic idea is in tact). Well I hope you get the idea. I'm sure you can thing of other things such as... (4) Oracle, Microsoft and others now allow you to query their databases in XML. So do companies (in my field) such as Apollo and Sabre. These are hardly compatible, nor do they in any way represent something that is comfortable to manipulate. An XSLT layer as a data abstraction layer allows you to transform someone else's standard into an internal one you can manipulate in a known way. (5) I mentioned XML is machine readable. Imagine: web pages are not very machine readable. Need I say more? I could go on, but I'm not an imaginative fellow. Let's see. There's also PDF. PDF isn't in XML but there is something called XSL-FO (formatting objects) which some PDF generators (perhaps the two that PHP has modules for?) understand. So writing XSL-FO for output for a PDF generator is "doing XML" also. Then there is the fact that it hierarchical data which sometimes, despite all these neat tools, needs to be parsed and understood. If we had a standardized API for manipulating it, then all the knowledge in learning the API for say visual basic on a Windows box can be transferred to doing in in C++ in AIX or perhaps PHP? Yes, there are two such standards, one is an event driven one (reads a tag and calls a callback function) known as "SAX" (Simple API for XML) and implemented in the Expat (--with-xml in PHP which is compiled by default in PHP4) and the other reads the whole thing into a hierarchical (treelike) object structure called the DOM (Document Object Model) which is implemented in libxml (--with-domxml or somesuch in PHP). I personally prefer the DOM version of looking at things (it's a bit slower and chews more memory). Unfortunately the dom-xml in PHP doesn't much resemble anyone elses DOM (at least not Oracle's or Microsofts), it's a bit buggy (for instance, you can't remove a node, nor can you seem to modify the text in a node) and it chews a whole slew of memory (much more than I'd expect and that amount has almost doubled since they've incorporated XPath support with PHP 4.0.4+). Now a final reason why PHP and XML should go hand in hand. (Because if you haven't figured out by now, I'm really big on PHP and on XML). A study I read estimates that by the end of this year over 50% of the Fortune 500 will be using XML in some "test bed" situation and by 2004 80% of all business communication on the internet will use XML. I hope this gives you some motivation to pick up XML and use PHP to do so. Because the more of us that are doing so, the more developers there will be working (or pushing others to work) on improving XML support in PHP. With more robust cool tools like the PHP developers have already given me (and hopefully will continue to do so), I won't feel so bad about studying condensed matter physics and neuroscience for the last 9 years instead of majoring in something real like computer science and learning how to code in Java and C++. Take care, terry chay -- terry chay, Director of Engineering, <http://www.QIXO.com/> QIXO /kick.so/ - Integrating Many Travel Web Sites Into One W: 1.408.394-8102 F:1.408.516.9090 M: 1.408.314.0717 E-Mail: <mailto:tychay@qixo.com> ICQ: 16069322 PGP Fingerprint: 6DCF 1634 547C 935D 4912 2A44 A4A2 79AB DFFF F110 -- PHP General Mailing List (http://www.php.net/) To unsubscribe, e-mail: php-general-unsubscribe@lists.php.net For additional commands, e-mail: php-general-help@lists.php.net To contact the list administrators, e-mail: php-list-admin@lists.php.net