This client links the WSDL file statically into their own source code. For the particular situation of phpVirtualBox this static approach works well, as each version of phpVirtualBox targets one specific version of the web API.
However, SOAP-based web APIs allow for the WSDL to be queried dynamically (Alonso et al., 2010,p. 186) and in a dynamic scenario where the web API client provides support for web APIs with different versions, this approach is recommended as a way to ensure the web API does indeed support the required version