SOAP, the Simple Object Access Protocol, is the powerhouse of
web services. It's a highly adaptable, object-oriented protocol
that exists in over 80 implementations on every popular platform,
including .NET, JavaScript, and PHP. It provides a flexible
communication layer between applications, regardless of platform
and location. As long as they both speak SOAP, a PHP-based web
application can ask a C++ database application on another continent
to look up the price of a book and have the answer right away.
SOAP was created collaboratively as an open protocol. Early in
its development, XML-RPC was spun off, and now enjoys its own
popularity as a simpler alternative to SOAP. Both encode messages
as XML, and both use HTTP to transport those messages. SOAP,
however, can use other transport protocols, offers a number of
high-end features, and is developing rapidly.
A SOAP transaction begins with an application making a call to a
remote procedure. The SOAP client script then encodes the procedure
request as an XML payload and sends it over the transport protocol
to a server script. The server parses the request and passes it to
a local method, which returns a response. The response is encoded
as XML by the server and returned as a response to the client,
which parses the response and passes the result to the original
function.
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