I just came across a really strange error while using PHP5's excellent SOAP functionality. My SoapClient was operating in non-WSDL mode but when I called function that did not exist I got the error:

Operation 'getFunctions' is not defined in the WSDL for this service

After spending quite a long time trying to get the client to realise it wasn't supposed to be using a WSDL, I realised I had mistyped the name of the function and what the error message actually meant was "this function does not exist". Hopefully I'll remember next time I see the same problem, hopefully by putting it here it might also help someone else too.