Interoperability is a very much highly mentioned feature when we talk about Webservice Framework for PHP. It s proven to be inter-operable with Not only WSO2 WSF/C, WSF/Ruby or WSAS, but also with Microsoft WSF and Sun J2EE in case of MTOM, WS-Security and WS-Reliable Messaging support.
It is no doubt interoperability is a must feature for a Webservice Framework. Because the idea of the webservices itself is to provide a medium to communicate between heterogeneous environments, So whenever you want to host a web service make sure it can be accessed from a client from any environment. In that case WSF/PHP have released PHP developers from these concerns by providing the interoperability from the framework itself.