Harry takes us one step further into the world of Web Services. Here he explains how to build your own Web Service – a news feed – using PHP and XML-RPC.
If you’ve been following industry news on the subject, you may think this is some high level technology, of interest only to big companies with huge budgets. But if you did, you’d be wrong.
The concepts behind Web Services are remarkably simple, and in this article we’ll be taking a deeper look at what’s involved. Then, with a little help from our good friend PHP, we’ll set up our first Web Service.
We will assume some knowledge of PHP, MySQL and XML, which — if you’re uncertain of — you can quickly pick up from Building your own Database Driven Website using PHP & MySQL and XML – An Introduction.
So here’s what’s on the menu:
The Basics of Web Services: To start with we’ll be quickly reviewing the basic concepts behind Web Services.
Introducing XML-RPC: Next, we’ll introduce you to an XML standard for the exchange of data between systems, and we’ll put it into context with Kevin’s article.
PHP XML-RPC Implementations: Then we’ll review some of the Open Source implementations of XML-RPC in PHP: code you can use to quickly build your own Web Service, or access other Web Services from your site.
Your first Web Service: If you want to get straight down to business, this is the place to be. Here, we’ll take one of the implementations described previously, and build a Web Service for it in PHP.
What you can do with XML-RPC: Wondering what to do next? We’ll give you some ideas for what you could do with XML-RPC and Web Services.
Let’s get started!
The Basics of Web Services
The first thing to understand about Web Services is they’re not really anything new. If you’ve ever used an RSS Feed to take news from another Website and place it on your own, you’ve already got a good idea of how Web Services work (see Kevin Yank’s article: PHP and XML: Parsing RSS 1.0).
Web Services are about exchanging data between a server and a client, using a standard XML format to “package” requests and data so that both systems can “understand” each other. The server and the client could both be Web servers, or any other electronic device you care to think of.
Network-wise, data exchange in a Web Service typically happens via TCP port 80, using standard HTTP protocol POSTs. Put another way, Web Services operate in basically the same way your browser does when it POSTs an HTML form to a site, and receives a Web page in response. The only real difference is that, instead of HTML, Web Services use XML. And this means Web Services can be available anywhere on the Internet, passing through firewalls the same way viewing a Web page does. The data exchange happens at the packaging layer.
On top of the data exchange, you also need information that describes the interface (or Application Program Interface – API) to the service. This makes the Web Service useful to the rest of the Internet, allowing other developers to develop programs that can access your Web Service. This is called the description layer, and the WSDL (Web Service Description Language) standard that will make this happen is under development.
Above that, there’s information that describes the nature of the service itself (not unlike the HTML-descriptive META tags), so that it can be categorised and found on sites that offer Web Service directories. This is the discovery layer, which is currently being addressed by the UDDI (Universal Description, Discovery and Integration) standard.
Both the description and discovery layers are simply XML, governed by a particular format that enables relevant information to be found for all Web Services on the Internet.
Perhaps what’s made Web Services a hot topic recently — aside from marketing by the likes of Microsoft and IBM — is the development of these standards. They’ll allow Web Services to be rolled out en masse across the Internet, backed by development tools that’ll make access to them both predictable and easy.
But it should be remembered that everything a Web Service does now, in terms of data exchange, could also have been done 5 or even 10 years ago using the HTTP standard and whichever XML format you chose to use or invent (RSS feeds being a prime example). The “hot news” today is that building and distributing a Web Service is now a lot easier than in the past.
Anyway, if you’re looking for a hype-free news source for the technological development in Web Services, try XML Hack.
We’ll skip further generalities, but suffice it to say that this article focuses on the packaging layer, i.e. how you build and access a Web Service.