The Zend Framework blog continues on its series of posts showing how to create various types of web services using various components from the framework itself. In this latest post they show you how to implement a SOAP server with zend-soap, a component specifically designed to "create, serve, and access SOAP applications, and parse and generate WSDL".
zend-soap provides a full-featured SOAP implementation. SOAP is an XML-based web protocol designed to allow describing messages, and, optionally, operations to perform. It's similar to XML-RPC, but with a few key differences: arbitrary data structures may be described [and] multiple operations may be described in a message as well.
The post goes on to talk about why they're show how to use these other service types when they primarily use REST in Apigility. It also covers some of the benefits using the module has over PHP's own SOAP handling. From there it's all about the code: first just creating the server and then populating it with the classes and functions it allows. The remainder of the post is split between two other methods for setting up the server: using it in a MVC application and as middleware in something like Zend Expressive.