Introduction

[EDIT] – This is a re-hash of a document I wrote a couple years ago.  There’s been changes to the nuSOAP library and I wanted to document the updates relative to the tutorial series.  Also, I need to fix a lot of the broken-links and code listings since I’ve changes hosting providers since this article was first written.

This article is specific to nuSOAP release 0.9.5 on 2011-01-13.  This tutorial was updated on September 22, 2011.

Once upon a time, I was tasked with developing a web-services by my boss as the integration point between our production application and Sales Force.  At this point, although I’d heard of web services … kind of … didn’t Amazon use web services for something?  Still, I’d never coded a web-services based application before.

At this point, I have to assume you are unfamiliar with the concept of web services and why you may have to create and provide a web-services offering to your client base.  Web services allow a remote client to access functionality (as defined by you, the programmer) via the standard HTTP interface (normally, port:80) to your application and database services.

Back in the day, networking services (semaphores, pipes, streams, message queues, and other forms of IPC) were custom-written and assigned/slaved to unique networking ports for accessing specific service daemons.  Of course, the internet was a kinder, gentler place back then… and a given server may have had dozens, or even hundreds, of non-standard ports open, listening, waiting, for networking service requests.  Or hacking attempts.

Today, web-services is a replacement to dedicated networking apps – all handled by your web server, and all serviced over the same network port: port 80.  Since port 80 is a standard port and, usually, already open on a web-server, additional security risks by opening new non-standard ports for networking services are averted.  Your web-server, such as Apache, now has the responsibility of processing the request and delivering the results to the client.

The web-services component piece is a collection of functions that you’ve written that provide remote clients access to your system.  These functions are accessible only via the web services framework and while they may be duplicated from a dedicated and traditional web-based application, the web-services framework is designed as a stand-alone piece of software.  Think of the web-services piece as your application’s data-processing layer minus the presentation layer.  The “M” and “C” of the “MVC” model.

Initially, when I was tasked with a similar set of objectives, I initially tried xml-rpc.  This led me down a rabbit-hole that spanned nearly a week of my time with the end-result being abandonment.  I hit road-blocks with xml-rpc over server authentication and passing complex objects.  Exacerbating the issue overall is that xml-rpc seems to be dated technology – I had a hard time locating resources that were recent.

Then I stumbled across nuSOAP – it’s free via sourceForge, stable and, using a quote from sourceForge:

“NuSOAP is a rewrite of SOAPx4, provided by NuSphere and Dietrich Ayala. It is a set of PHP classes – no PHP extensions required – that allow developers to create and consume web services based on SOAP 1.1, WSDL 1.1 and HTTP 1.0/1.1.”

nuSOAP seemed to have more of everything available: tutorials, examples, articles, blog posts.  When I started my implementation with nuSOAP, the first thing I received help with was server-level authentication.  I was able to immediately get my remote requests validated by the web-server and handed off to the web-services module!

The major selling point, for me, on nuSOAP is that nuSOAP is self-documenting.  As part of the API functionality, nuSOAP generates HTML pages that documents the exposed services via the WSDL  and also provides you with a complete XSLT definition file!

First off, download and install the nuSOAP libraries – I provided a link to the sourceForge site a couple paragraphs ago – and unpack the tarball.  You’ll end-up with a directory (mine is called: ./nuSOAP) and, within that directory, is the one file you include: nusoap.php.

There are two pieces to this tutorial — a server side piece and a client-side piece.  While you can execute both pieces off the same environment (machine), normally you’d use the client remotely to access the API server-side code.

What’s Not Covered:

Apache.  Apache configuration for your vhost.  Apache .htaccess.  This article assumes that you’ve a working server and that you’re able to install and access the server files via Apache.  Even if your Apache server is a localhost configuration, access the server-side files via localhost client, traversing the TCP stack locally, is still a valid method for testing your web-services server application.

Time to push up our sleeves and start working on the server code…

The Web-Services Server

Today, we’re going to write a ping server — where the server has an exposed service (method) named “ping” which takes a single argument (a string) and returns an array back to the calling client.  The return array contains two associative members: a boolean (which should always be true – otherwise there are other issues…) and a string which is the modification of the original string in order to prove that, yes, we went there and we came back.

Because my project, and I’m doing this project for my new company, is going to represent significant effort, size and complexity, I’ve broken out the components of nuSOAP request into exterior files because, later, these will become control file which will, in turn, load files that have been organized into a hierarchy friendly to the application’s data model.

So, if this file, which I’ve named index.php, seems small, remember that you’re not viewing the dependent files (yet).

So far, so good – let’s take a look at what we’ve just done:

• we’ve included the nu_soap library…
• declared our namespace (which is the URL of the api server)
• instantiated a new soap_server instance and assigned it to the variable $server
• initialized the WSDL
• assigned the namespace variable to the WSDL
• load and register our exposed services
• load the WSDL
• load the service code
• create the HTTP listener
• invoke the requested service
• exit

This (index.php) file is the server-side file that will be invoked for ALL future API calls to the service.  It invokes three control files which, in turn loads the services (WSDL definitions), the WSDL variable definitions (think of these as inputs and outputs to your exposed services), and the actual code for all of the services, and their supporting functions, that you’re going to expose via your API.

Side Note:  This is the file you’ll reference in Apache when you’re (preferably) creating a new Virtual Host for the API.  HTTP requests that resolve to your server will be serviced by Apache which will, in turn, serve the results of this program back to the client.

Next, we’re going to define the myServiceRegistrations.php file that is required by index.php.  This file contains the WSDL for each and every exposed service that the API serves.

This PHP code registers a function called “ping” with the nuSOAP $server instance.  The second parameter is the input to function.  Note that all input (and output) parameters have to be declared as an array even if there’s only a single value being passed.  Also notice that you have to type-cast the variable being passed using XML datatypes.  For your data definitions, you use one of the 44 built-in datatypes defined in this document: http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/.

(For more information on XSD object and XML schema, please visit: http://ws.apache.org/axis/cpp/arch/XSD_Objects.html.)

The third argument to the method “register” is the data type returned.  This is a complex variable called “aryReturn” — don’t worry about right now, we’re going to define this complex-type variable in another configuration file.

The next four arguments are:

• our $namespace variable we set in index.php
• boolean false
• ‘rpc’ for the call type
• ‘encoded’

Nice, huh?

The second file (that we’ve included in our source: (mywsdl.php)) is the WSDL file that defines our data structures that are used as either inputs, outputs, or both, to the exposed services.  Another thing I like about SOAP and nuSOAP is that it introduces a layer of strong-typing to the PHP stack.  You can save a bit of debugging time by requiring that you call a method with EXACTLY this and return EXACTLY that.  Anything else tends to generate a system-level error:

This error message, from the apache error log, is somewhat obfuscated in it's meaning.   I attempted to return only the string, by itself, instead of returning the array (of two elements) that I had told nuSOAP I would return for this service.  This error was generated because the types (between the code and the WSDL) of the return variable (structure) did not exactly match.

If you've only ever coded in a loosely-typed language, like PHP, than this part of SOAP is going to be a bit of a ... transition ... for you.  When we say that something, be it a variable, function, or exposed service, is strongly typed, we're declaring the type of that object and, if the type of the object during run-time does not match, then SOAP will force PHP to throw a fatal as shown in the error log snippet above.

Keep this in-mind as you develop exposed services that are increasingly complex.  Since the error messages tend to point you at your code, at the point of failure, it's easy to forget that that the requirements of the underpinnings (in this case, the WSDL), are the root cause of your PHP fatals.

That being said, let's take a look at the WSDL for our ping service:

We're invoking the nuSOAP method addComplexType to define a structure to the WSDL in our Table Name Space (tns).   To do this, we first define the name of the structure that we're going to use: aryReturn and then we define the composition of that structure.

The declaration for this looks a lot like a standard PHP declaration for an array with the exception of the XSD (XML Schema Definition) appended at the end of each element's declaration.  (See the links I embedded above for explanations and examples of valid XSD.)

XSD provides part of the strongly-typed concept for our structure elements.  We're telling nuSOAP to expect a variable structure containing these named elements of this type.

What we have, then, is an associative array with two elements: 'status' and 'data'.  $aryReturn['status'] and $aryReturn['data'] and they're of type BOOL and STRING respectively.

Note, finally, that this variable structure isn't confined to single-use.  Once we've declared it within our tns, it's available to any exposed service where it's needed.  This is the model for my common error structure -- the boolean indicates success or fail on some service operation and the data component contains the relative diagnostic information.

The third and final file we're including into the server source is the code for the exposed service.  This is where you write the function handlers for your services.  Since you've already defined the input parameters, and the return types for the ping service, there's very little left to do.

Note the following in the code for our exposed service:

• our exposed method is named ping because that's how we registered the service (myServiceRegistrations.php)
• we're providing a default type-cast for the input param of string in case the service is invoked without input params.
• we're returning BOOL true and prepending "Rcvd: " to the received string, and appending a character count to prove that the service successfully responded to the client's request.
• the return structure exactly matches the WSDL declaration: the name of the array elements, and the element types.

If you're using IE, then clicking on the WSDL link will return the XML.  If you're using Firefox, Chrome or other browsers, clicking on WSDL will display the generated XML for your service.

Now that the server is working on it's own, it remains fairly useless until we can get a client to connect to it invoke it's methods.  Let's work on the client next...

The Web-Services Client

The web services client application will also be written in PHP.

The web client is an application that connects to remote server using the http port 80.  To do so, you'll need the client to be aware of certain bits of information that may, or may not, be required to access the remote server.

In our client, we're not going to require remote authentication -- but I'll take a quick aside andexplain how you would include this, client-side, if your server required .htaccess authentication.

nuSOAP has a client method called setCredentials which allows you to specify your .htaccess username and password and the authentication schema.  It's a single line of code which is normally used to require not only clients to login to access your API, but, once identified, you can limit the set of exposed methods available to individual clients or groups.

For example, if you have a product you've developed in-house, then you'd want full-access for your front-end web/applications servers.  Your PM later decides to open a subset of the API to the general public and a subscription-based set of exposed methods in order to monetize your product.  Finally, the PM also wants to "white-box" the product so that other companies can use it but with their branding and access to isolated or discrete data sets.

What you'd end-up with is several levels of client access to your web services.  Implementation of limiting exposed services would be handled server-side but it would be based on your client's authentication and possibly the subject of a future tutorial...

So, to the client-side code: (name this file: apiTestClient.php)

The first thing we do in our client-side code is to include the nuSOAP libraries.

The next five lines of code read from the local POST environment, testing if you've established a proxy for your web-services server and, if so, populating the proxy variables.

The next line instantiates a nuSOAP client and associates it with our remote server.  Change "YOURSERVERURLHERE" to the name of your apache web server URL where you have the server side code installed.  (e.g.: localhost, myserver.com, etc.)

Note the name of the function call: newsoap_client()...as opposed to using the function soapclient().  This function name is legacy-compatible with PHP 5.0's instantiation call:  new soapclient() - the PHP SOAP extension uses the same instantiation function name as the nuSOAP library.  If you have both installed, (PHP 5.0 SOAP extension, and the nuSOAP libraries), executing the client will return errors as you've overloaded the soapclient() function.  (You're calling the PHP SOAP function with the nuSOAP function parameters.)  Rename the soapclient() function to the back-compatible function: newsoap_client().

The next two lines tell the nuSOAP client to useCurl, when possible and if previously saved and, if possible, to use HTTP persistent connections.

Next, we're going to consume the web-services from the server by making a call to the nuSOAP method: call().  The arguments to this method are:

1. the name of the service being consumed (ping)
2. the input string (note:  all inputs must be passed as arrays!)
3. the namespace URI (optional:  WSDL can override)

Store the results of the web-services to the aptly-named variable $results and evaluate it upon return.  If the client call was not successful, then display an error message.

If the client call was successful, then display the contents of the returned array.

Note that you can run this client code from either a browser using the "file://" option or, if the client source code is accessible to apache, then you can display using a browser.

When I run this client-side software in the browser, I get displayed back:

Array ( [status] => 1 [data] => Rcvd: Argle(5) )

So, how do we know that the client went out and successfully returned from the server with the data?  Simple - in the client source, you will not find the literal "Rcvd:" anywhere in the source.  This data was supplied by the server-side function and returned back to the client.  It's a simple example, but it provides proof-of-concept that we're successfully able to connect to a remote web-server and return data created on the remote server back to the client program.

Let's wrap this up...

Summary:

This tutorial (hopefully) explained what web-services are, and provided you with a practical example of a consumable service: ping().  Such a service would normally be invoked as a means of testing server availability.

We created a web-services server file using the nuSOAP library by defining a complex-structure (an associative array) and registering a method with the nuSOAP server.  The method takes an input parameter which, although it's only a single input parameter, must be built and passed as an array construct to the server method.  Next, we showed that by loading the server source into a browser, we learned the nuSOAP provides built-in documentation for your web-services structures and methods.  Which is a very nice-to-have when you're creating your developer documentation!  Next we created the web-services client source code file which connected to our remote server and invoked the server's method: hellow().  If all worked correctly, you displayed the string returned from the remote server within your browser window.

In the next installment, I'll cover the web-services server-side of this business and we'll see how to create the various methods for accessing a mySQL database, complex structures as input and output parameters to those methods, and general debugging techniques.

Thank-you for your patience - I hope this article helped you.

September 26, 2011