Soap (Simple Object Access Protocol)
SOAP  is a lightweight XML-based protocol for exchanging
structured information between distributed applications over native web protocols,
such as HTTP. SOAP specifies the formats that XML messages should use, the way in which
they should be processed, a set of encoding rules for standard and application-defined data types,
and a convention for representing remote procedure calls and responses.

NuSOAP library  
It is a set of PHP classes that allow developers to create
and consume web services based on SOAP 1.1, WSDL 1.1 and HTTP 1.0/1.1.
You can download the latest version from the above link and copy the nusoap.php
file into your web directory. Afterward you can include this PHP file in your code.

What you need to know about WSDL is that it is a document that describes a Web Service.
It can tell a client how to interact with the Web Service and what interfaces that Web Service provides.

The NuSoap library which we will be using to write our web service generates the WSDL for our web service.

In a WSDL file, you specify the interfaces to use.
Sophisticated SOAP programming tools can read a WSDL file and automatically define
functions described in the WSDL file, which you can call like any regular function,
and it will transparently do the query through a network connections.

The WSDL file is an XML file, that consists of five sections:

    Basically a list of typedefs for use in parameter and result passing.
    The types may refer to each others forming tuples, arrays and trees,
    or to XSD types such as xsd:string and xsd:int.
    The structure of each query and their relevant response.
    For queries, the parameters are listed. For responses, the return values are listed.
    The names of the messages are internal to this WSDL file.
    Declares a port by a name, and lists the names of
    function calls (operations). For each function, it describes which message
    is used for calling (input) and which message is used for response (output).
    It describes the method of passing the input and output for each operation.
    Typically, they are encoded in SOAP encoding, but they can also be passed raw.
    For each port declared in portType, it describes the endpoint (address) through which the query is performed.

    to read more on how to write a WSDL file visit :´╗┐