I spent quite some time looking for info on how to specify the xsi:type for the objects, and I finally came across SoapVar. I created a base class which the SOAP classes extended. A method called pack is responsible for setting xsi:type.

In his example, he creates a BaseClass to work from and builds a createCustomer class on top of it, defining a setCustomer function inside. When the new Person is created, a new Customer is too complete with the correctly formatted type on the object.

How do you get this [custom] XML to be output by the SoapClient?

Using the SoapVar method and a few newly created objects, making a custom structure is simple. It's just a matter of encoding the object and setting it with the base() method in the PHP SOAP extension. Check out the blog post for an example of both the custom XML and the code to produce it.