PHP and Web Distributed Data eXchange (WDDX)
"Beam me up, Scotty!"
~Captain James T. Kirk, "Star Trek"
The preceding chapters have focused primarily on parsing XML documents with a strong emphasis on producing content for web browsers. Although accomplishing this is no mean featin fact, it's one of the most popular ways to use the XML/PHP comboit's just the tip of the XML iceberg.
You'll remember from my opening remarks that XML provides constructs to encode any type of information in a standard, machine-readable format. This makes XML the ideal vehicle for information exchange over the web. All that's needed is an encoding format that is understandable to both sender and receiver and that can piggyback over standard Internet protocols (HTTP, SMTP, FTP, and so on).
That's where the Web Distributed Data eXchange (WDDX) comes in. WDDX provides a standard format for creating XML-based data structures designed for easy transmission across the Internet. These WDDX data structures are largely platform-independent, and they can be decoded and used by any application that understands the WDDX format.
Over the next few pages, I will be examining WDDX in greater detail, demonstrating how it can be combined with PHP to encode and exchange data across different systems and platforms.
This chapter marks the transition from merely parsing XML data to actually using XML as the vehicle for other applications. In addition to detailed descriptions of how PHP can be used to create WDDX structures, I'll also be demonstrating some real-life applications of the technology to illustrate its usefulness and versatility.
Invented by Allaire Corp. (makers of the HomeSite HTML editor and the ColdFusion application development environment), WDDX is " . . . an XML-based technology that enables the exchange of complex data between web programming languages . . . 1 It was created in 1998 as an open standard designed specifically to simplify data exchange across different platforms, and it has quickly gained popularity with web developers for its elegance and ease of use.
WDDX works by converting language-specific data structures into their corresponding XML representations. These XML data structures are text-based, platform-independent entities, and, as such, can be transmitted between different systems over standard HTTP protocols with minimal difficulty. Any WDDX-friendly application can read these WDDX packets, and convert them back into their original form. For example, a Python list could be encoded into WDDX and transmitted across HTTP to a PHP script, which could decode it into a PHP array. Or a PHP associative array could be translated into WDDX, sent to a Perl script, and decoded into a Perl- compliant hash, for use within a Perl script.
Perhaps an example would make this clearer. Consider Listing 5.1, a single-line PHP script that defines a variable and assigns it a value.
Listing 5.1 A Simple PHP Variable
<?php $str = "amoeba"; ?>
If this variable were to be converted (or, as the geeks say, "serialized" or "pickled") into its WDDX representation, it would look something like Listing 5.2.
Listing 5.2 A WDDX Representation of a PHP Variable
<wddxPacket version='1.0'> <header/> <data> <string>amoeba</string> </data> </wddxPacket>
As you can see, this is regular XML markup, with both the variable type and its value embedded within it. The document element here is the <wddxPacket> element; it can be separated into distinct header and data areas. The header contains a human-readable comment, whereas the data block contains an XML-encoded structure representing the data to be transmitted.
This WDDX representation can be decoded (or "deserialized" or "unpickled") and used by any application that understands WDDX. And so, by creating a standard framework for representing common data structures and by expressing this framework in XML, WDDX makes it possible to easily exchange information over the standard Internet backbone.
It's important to note that WDDX is not an "official" specification per se; rather, it's an open standard created and supported by one company. Despite this, it's fairly popular, primarily for use in client-server or server-server content publishing systems, or as a wrapper for data exchange between multiple programming languages. By allowing applications written in different languages to easily communicate with each other, it also opens the door to new B2B applications, particularly in the areas of streamlining business processes and transactions.
For more information on WDDX, you should refer to the official web site at http://www.openwddx.org/, which contains the WDDX Document Type Definition (DTD), an SDK (if you need to implement WDDX on an unsupported platform), and usage examples.
Not My Type
The WDDX specification currently defines the following data types:
Boolean values, represented by the <boolean> element
Numbers, represented by the <number> element
Strings, represented by the <string> element
Date/time values, encoded in ISO8601 format, represented by the <dateTime> element
Integer-indexed arrays, represented by the <array> element
Structures, or string-indexed arrays, represented by the <struct> element;
Recordsets, or two-dimensional (rows versus columns) data collections, represented by the <recordset> element